We are used to the decimal number system, i.e. the base 10 number system. As we all learn in school: 546 is five hundreds, four tens and six units:
$546 = (5 \times 100) + (4\times 10) + (6 \times 1) = (5 \times 10^2) + (4\times 10^1) + (6 \times 10^0).$
The base 10 refers to the fast that we write our numbers as a combination of powers of 10. In the base 18 number system, we would have:
$ 546 = (5 \times 18^2) + (4\times 18^1) + (6 \times 18^0).$
Things get a bit messy though. In base 10, we need 10 symbols: $0, 1, 2, \ldots, 9$ for each position. We only go up to 9 because 10 units is one ten, ten tens is one hundred, ten hundreds is one thousand, etc. In base 18 we would need 18 symbols for each position, e.g. $0,1,2,\ldots,9,A,B,C,\ldots,H$. An example of a number is base 18 could be:
$1H2E = (1\times 18^3) + (17 \times 18^2) + (2\times 18^1) + (15\times 18^0).$
Of course, the famous example is binary, which is base 2.