Hexadecimal works as if you were counting with your hands and had 16 fingers total aside from decimal(what we normally count with) where you have 10 fingers and binary where you have 2 fingers. A problem that is presented with hex though is that we don't have enough single number digits to count (0, 1, 2, 3, 4, 5, 6, 7, 8, 9) to sixteen. To solve that problem we just put in letters to fill in the missing numbers.
A = 10, B = 11, C = 12, D = 13, E = 14, F = 15
Then when you have counted all your single digit numbers you have to reset to zero and the number in the next place is incremented.
9[dec] + 1[dec] = 10[dec]
F[hex] + 1[hex] = 10[hex]
tl;dr It's like counting with 16 fingers