Numbers are a thing on their own, independent of how we write them. We normally use decimal notation, with positional value, ie, first place is units, second place tens and so on.
Decimal means that we have ten symbols and once you reach the largest value symbol, 9, you roll to the next position.
Hexadecimal is base sixteen, so the first position goes up up to fifteen before rolling over. To represent this we use A for ten, B for eleven and so on. F represents fifteen, so 10hex is 16dec, and FFFhex is 4095dec. To convert FFF to decimal you can do 15 × 16 × 16 + 15 × 16 + 15.
Hexadecimal is used in computing often, as a more compact way of writing binary, which is base two, as sixteen is two to the power of four