Let's say you had the ratio 14:21
The GCF of the two values (14 and 21) is 7
14 = 7*2
21 = 7*3
We can divide both parts of the ratio by the GCF 7 to get
14/7 = 2
21/7 = 3
Meaning the original ratio 14:21 fully reduces to 2:3
So in short, we use the GCF to help fully reduce any ratio.
This also applies to fractions as well. We could have something like 9/33 fully reduce to 3/11 after dividing both parts (9 and 33) by the GCF 3.