The base of a numeration system refer to the total number of digits that are used in order to represent every single number in that numeration system.
For example, the base in the decimal numeration system, also called base-10 number system, is 10. The base-10 number system uses the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 to represent every single number in the decimal numeration system.
The base in the binary number system, also called base-2 number system, is 2. The base-2 number system uses the digits 0 and 1 to represent every single number in the binary number system.