A shield has always meant some sort of barrier for protection. Think 2000 years ago of something large and flat you hold so that the other guy can't clobber you with his sword.
In electronics, the term shield means a conductive layer that prevents capacitive coupling to whatever you are trying to protect with the shield. A common example is found in typical audio cables. These are coaxial meaning both conductors are symmetric about the same axis. The signal-carrying conductor is a ordinary wire in the center, and the ground conductor is a hollow cylinder around it, with insulation between the inside of this cylinder and the center wire. External (noise) signals to the cable can capacitively couple to the outer ground wire. The little bit of current picked up there thru the capacitance to the noise source is harmlessly shunted to ground. The inside conductor only sees the 0 V of the ground shield all around it and doesn't pick up the noise.
In electronics, shields also work in reverse. We sometimes put a conducting shield layer around something with lots of noise voltage. This heads off the touble it might cause at the source.
The Arduino Confusion
Arduinos are just microcontroller development boards with software and documentation that strive to let people get simple things done simply without having to know much about microcontrollers, firmware, or electronics. So far so good.
The problem was caused by arduino marketing taking this a step further. I guess they felt the artists who were their target customers at the time might run away screaming when confronted with a "daughter board" or "plug-in board". Their solution was to call such things a "shield". The fact that this word already had a established meaning, and even a more specific meaning in electronics was of no consequence to them. "Screw you, it's not our problem."
In addition to appropriating the word "shield" for their own personal gain at the expense of everyone else, they also did this with the word "sketch". Again, they apparantly felt the few neurons devoted to technobabble in their artist customers' brains would overload at thinking they were writing software, or (gasp!) actually programming. Ewe! The solution? "You're just creating a sketch. Nope, no programming here. Never mind that what you write looks a lot like something those silly knurds call C.".
As you can imagine, this arrogant attempt to repurpose existing words for the personal gain of the arduino sellers at the expense of everyone else pisses off a lot of people. This is particularly true of electrical engineers, for whom "shield" already has a special meaning specific to the discipline. As a result, those using "shield" in the arduino context will be met with much hostility here. I personally am likely to downvote any such question, and look for a handy excuse to close it too. And, I'm not the only one.
Arduino Questions Don't Belong Here
This section is really a meta discussion, but it needs to be said within the context of the above.
This site (EE.SE) is for electrical engineering. That means we are all about understanding circuits, components, and how they work. We are happy to help others who are trying to learn this too, or have particular problems in the course of doing electrical engineering.
Note that arduinos were specifically created for the opposite, which is getting results without having to understand the underlying technology. Again, that's all fine by itself, but this site is about the underlying technology and helping people who want to learn it. It is absolutely NOT for "i dunno, jus plug dis inta dat and it work".
Therefore, asking about something related to a arduino here is basically saying you just want to get results and don't want to be bothered with all those silly 'tronics thingies. Just gimme da codz! Naturally that will result in the digital equivalent of getting tarred and feathered while run out of town.
How to Ask Arduino Questions Here
This may seem odd since I just got done saying that arduino questions don't belong here. However, electronics questions that happen to come up while doing something with a arduino can be fine. The point is these questions must
- not be about the arduinoness,
- be about a real electronics problem,
- and exhibit a desire to learn about the underlying technology, not just to get a particular result now.
Since "arduino" in a question will be read as "I don't wanna know how this stuff works, just gimme da answer so I can go back to laughing at all you silly techno-weenies behind your backs. Hey, if I actually respected you lot, I wouldn't barge in here flagrantly misusing your specialized words, and maybe I'd bother with one of those skmatic thingies instead of this wires diagram some software I found on the internet made for me. So fork over the answer already, you insufferable knurds.", it would be wise to leave it out. Put another way, by mentioning "arduino" at all, you have shifted the presumption of what you are here for. The burden of proof is now on you, and the bar will be set higher. Such things happen when you piss people off.
So what to do? Don't mention "arduino". Not only won't that immediately label use a one of those annoying ardweenies, but it's a good test for you whether your problem is appropriate here. If you could just as well say "microcontroller" or "microcontroller development board", then you probably do have a general electronics questions. Of course then there is no need to actually mention "arduino", so don't. If the fact that the problem is related to a arduino is important, then it's a strong clue that it's a arduino user-level issue and doesn't belong here.
So in summary, first get rid of all mention of "arduino" and related usages of "shield" and sketch", then
- If the question still makes sense, you have a general electronics question. Ask it here.
- If the question no longer makes sense, then it doesn't belong here. Go away. We don't give a crap where to (although there is now a Arduino.SE site where your question may be a fit).