The GCF is the greatest common factor, or the biggest number that both numbers can be divided into. Let's break it down!
44 = 1 x 44, 2 x 22, 4 x 11.
66 = 1 x 66, 2 x 33, 3 x 22, 6 x 11.
Both of these numbers have a common factor of : 2, 11, 22.
The greatest common factor is 22.
I hope this helps!