The GCF means Greatest Common Factor. This is the highest possible factor that multiple numbers have in common.
For example, 5 and 10.
5 = 1 x 5
10 = 1 x 10, 2 x 5
They both have 5 as the highest common factor, so 5 is the GCF.
Now that I've broken down what GCF means, let's factor out our numbers.
52= 1 x 52, 2 x 26, 4 x 13.
78= 1 x 78, 2 x 39, 3 x 26, 6 x 13.
These numbers both have 13 as the highest number.
The GCF of 52 and 78 is 13.
I hope this helps!