Prime factorization is breaking down a number into factors until it is composed/simplified into only prime numbers.
For example, 32.
32 : 2 x 16, 4 x 8
Now break down the factors.
2 x 2 x 8 (16)
2 x 2 x 2 x 2 x 2 = 32 in prime factorization.
Now that I've explained this (please ask questions in pm if you need me to explain it better), we can find the prime factorization of 84.
84 = 2 x 42
2 x 2 x 21
2 x 2 x 3 x 7 = 84.
2 x 2 = 4
4 x 3 = 12
12 x 7 = 84.
The prime factorization of 84 is:
2 x 2 x 3 x 7
I hope this helps!