The greatest common factor is the a term that you can take out of all the terms given to you. This term, when multiplied to each of the individual numbers in the set, will return you to the original amount.
In this case, note that they all share the variables y and z, and that each of them have <em>at least</em> 3 y's and 3 z's. For you to factor, you will divide these from all the terms.