I read a textbook and it states that:
"The effects of electrical noise can be minimized using circuitry external to the MCU"
I don't understand how this scenario works. What does it mean with 'external circuitry'?
If the question was targeting also the susceptibility to external noise, than the answer accepted was not complete. There is much more involved here.
I can recommend an excellent book on the subject, The Circuit Designers Companion.
You will want to read at least the first two chapters, grounding and wiring.
Decoupling capacitors have their role in reducing the electrical noise radiated out by the circuit. Narrow but possibly high current power supply peaks are contained within the small area near the high speed components, instead of pulling the current all the way from the power supply.
However, if the question was also how to prevent the EMI (Electromagnetic Interference) from the outside to play havoc with your circuit, that there are many other factors involved.
One of the most important things you should take care of is the cable and signal routing. The ground references should be kept separated, and if you had several circuits boards their grounds should be connected in s single point (star topology grounding).
High speed or high current lines should be kept separated from the low level signal lines. If such cables (or PCB traces) have to cross their paths, it should be done at right angle, minimizing the length of path running in parallel, and forming a stray capacitance.
Analog and digital inputs should be protected by filter components, and protection diodes. Output components switching high currents with inductive loads should also be protected by schottky diodes and filter components. Very often the software plays important role. For example, some communication protocols can adjust the signal slew rate (signal edge rise / fall time) to reduce the radiated interference.
There are many other measures, besides obvious shielding, keeping the 'electrically dirty' parts away, orienting the transformer so then it does emit it's magnetic field through the low voltage input stages of some sensitive amplifier. Avoiding or at least keeping the signal path loops short and narrow is always a good practice. Some beginners would route the PCB in a way that there is a power supply (or ground) trace around a board, just in case something needed to be connected. It it is fine if this was a true ground plane, but if it is just a wider track then it should be broken at some point, or it will serve as an antenna (both receiving and transmitting noise). I hope you have the picture, this subject is broad and involves much, much more than spreading few capacitors around the board.
Your textbook is probably talking about decoupling capacitors. It looks like this:
A small capacitor (most of the time 100nF) placed near the MCU from VCC to VSS. It filters away all the high frequencies, so that no noise gets to the MCU.
Why not a large capacitor? Basically because it doesn't respond that good on higher frequencies: What's the use of a decoupling capacitor near a reservoir capacitor?
The red capacitors are decoupling capacitors. As you can see, two different values are used. This is to eliminate as much frequencies as possible.
The blue capacitors are DC-blocking (AC-coupling) capacitors and are used to shift the center of the AC (RF) signal back to ground to match the dynamic-range of the receiving system. They are sized to minimize attenuation of the desired signal by presenting as low an impedance as possible at the signal's frequency band.