A number base (also called radix) tells you how many digits are available before you "carry over" to the next position.
In base-10 (decimal), you have digits 0 through 9. After 9, you carry over: 10, 11, 12. In base-2 (binary), you only have 0 and 1. After 1, you carry over: 10, 11, 100. The base determines how many symbols you use.
Each position in a number represents a power of the base. In decimal 523, the 5 means 5 × 10², the 2 means 2 × 10¹, and the 3 means 3 × 10⁰. This pattern works for any base.