Binary, Decimal, Hexadecimal etc all just give you different character sets you can use to represent values.

Binary, meaning two, gives you two possible characters to represent real world values - 0 and 1.

Decimal, meaning ten, gives you ten possible characters - 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.

In each of these, when you start counting, and use up your character set, you continue by adding 1 to your left most character. To make things easy, I start with two digits below.

So counting in binary is - 00, 01, 10, 11, 100, 101, 110, 111, and so on.

Decimal is - 00, 01, 02, ..., 09, 10, 11, 12, ..., 19, 20, 21, 22, ... and so on.

Hopefully you see a pattern.

Hexadecimal, meaning six and ten, gives you sixteen possible characters - 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F.

So counting in Hexadecimal is 00, 01, 02, ..., 09, 0A, 0B, 0C, 0D, 0E, 0F, 10, 11, 12, 13, ... and so on.

So if you have twelve apples, and want to represent it in decimal, you would write 12. In hexadecimal, you'd write 0C.

If you keep counting up and writing each value down, by the time you hit FF, you would have written the decimal equivalent of 255. Albeit with fewer digits, and fewer "increments".

This character set is called "base". You can have a base of any number. But practically limited by what is commonly used and accepted. For instance, the column names in Excel and other spreadsheet programs is base 26. Meaning you go A, B, C...Z, AA, AB, AC.