This is the GCF of 18 and 12. We can factor each into prime factors like so
18 = 2*3*3
12 = 2*2*3
In each factorization, we see exactly one copy of '2' and one copy of '3'. These values are in both factorizations at the same time. So that is why the GCF is 2*3 = 6. It is the largest whole number that goes into both 18 and 12.
Note: we ignore the second copy of '3' because 12 does not have two copies of '3'. A similar story happens with that second copy of '2'. GCF stands for "greatest common factor".
