Binary is a base-2 system. Each place is marked by either a 0 or a 1.

00000001 = 1
00000010 = 2
00000011 = 3
00000100 = 4

Place values from right to left: units, 2, 2 squared, 2 cubed, etc.

It's used in computing because it's a series of on/off switches that are easy to calculate electronically. It's the root of bits and bytes.