A bit (short for “BInary digiT”) is a single on/off value. Only these two values are possible, though the two values may go by different names, such as true/false, or 1/0. There are many ways in which a bit can be implemented. Here are some ways that bits are implemented:
A mechanical electrical switch (like a light switch.)
Voltage on a wire.
A light bulb being on or off.
The state of a single transistor.
The magnetic charge on a tiny part of the surface of a disk.
The presence or absence of a hole punched in a card.
A tiny part of the light-reflecting surface of a CD.
Part of a radio signal.
Computers do all of their processing in terms of bits - any piece of information a computer stores or manipulates is stored as a sequence of these on/off values. For different purposes, a computer may use any of the mechanisms listed above (or many others) to represent those bits.
Introduction to Computer Science using Java by Bradley Kjell (CC BY-NC 3.0)