Number a piece of paper vertically from 0 to 15.
Suppose that rather than ten digits (0 - 9), we only had two: 0 and 1. Now, starting with zero, begin counting, writing your numbers down. Use the same patterns that you observed when counting using the ten usual digits. Write the number down next to the numbers 0 - 15 you already wrote.
The numbers you just wrote are called binary numbers. You sometimes hear them called base-2 numbers. Indeed, they are part of a base-2 number system, just as the number system that we usually use is base-10. Instead of digits, binary numbers have bits.