Base two numbers are displayed in binary form (i.e. 0 or 1) as opposed to base 10 (normal) numbers.
A simple example of this would be the number 10 itself. This is a base 10 number but when shown as a base 2 number, it would be shown as 1010 - from right to left - 0 units or 1's, 1x2, 0x4 and 1x8.
Hope this is helpful. If not you could check out any of the GNVQ on-line maths sites (there are lots) which will explain in more detail for you.