Binary Counting Digits - 1
A switch can be either OPEN, or CLOSED. It has only TWO states - OFF, or ON.
In the case of a computer, when the switch is ON five volts appears on the line. When the swicth is OFF, zero volts appears. Five volts is also termed: HIGH and zero volts is termed: LOW.
The word "binary" means two. Computers are said to operate in "binary" mode, meaning the values inside the computer can be one of two states.
A HIGH state is indicated with the numeral "1". A LOW state is indicated with the numeral "0".
Data is passed around the computer as a group of 1s and 0s. A typical digital "word" might look like: 0001100011100010. This particular word has sixteen places and wold be termed a "16 -bit" word.