The greatest common factor (a.k.a. GFC) is the highest number that can divided by a given number to equal an integer, but the GFC can't be the given number.
For example, lets say you were given the number 100. What is the biggest number 100 can be divided by and will not give a decimal?
50 is the greatest common factor.
There are some number that don't have greatest common factors, like 13 and 5 because they're prime numbers. Prime number can only be divided by the same number.
