The base means how high you can count using only one digit. In base ten you have 0-9. In base one thousand, 0-999 are all represented as a single digit (you'd need a massive numerical system). Let's say that w is the number for 998, and x is the character for 999. You'd count 0, 1, 2, 3...w, x then the next number (one thousand) would be 10. 11 is one thousand and one and so on all the way to one thousand nine hundred and ninety nine which would be 1x, then two thousand would be 20.

The base is just how high you can count before you need a second digit. In base 2 you can only count 0 and 1 before you need another digit. That's why two is written "10" in base two.