The GCF is the greatest common factor. The GCF of the numerator and the denominator is what you need to divide each by in order to reduce the fraction to lowest terms.
Example
125/175 = (5*25)/(7*25)
The greatest common factor (GCF) is 25. Dividing numerator and denominator by 25, we get
125/175 = 5/7
Example
125/175 = (5*25)/(7*25)
The greatest common factor (GCF) is 25. Dividing numerator and denominator by 25, we get
125/175 = 5/7