meaning of set low/set high

I often see those phrases in tutorials with microcontroller I/O. An example use is "set pin 3 high", or "set pin 5 low for 20 ms".

I also recall reading several different definitions on what that terminology means. Does the low in "set low" mean low impedance? Does it mean low logic level (0 volts)? Or does it mean low as in negative voltage? Or is there even a universal meaning for those phrases?

• This link will be useful for you. – tyblu Dec 15 '10 at 8:00
• @tyblu thanks, but I'm already pretty familiar with binary and the actual software side. I was only confused about how a binary value is communicated electronically to another device. – Ponkadoodle Dec 15 '10 at 18:21

• The d in Vdd refers to the drain of a MOSFET (in CMOS), the s in Vss to the source. You'll often see Vcc instead of Vdd. Here the c is for collector (of a bipolar transistor), a remainder from the older TTL (transistor-transistor logic) circuits. – stevenvh Dec 15 '10 at 10:14