factors are the numbers you multiply together to get another number. when you find the factors of two or more numbers, and then find some of the numbers (factors) are the same (common) you have common factors.
example: 12 and 36
12: 1x12, 2x6, 3x4
36: 1x36, 2x18, 3x12, 4x9, 6x6
the common factors in these numbers are 1, 2, 3, 4, 6 and 12
hope this helped you out!
