Originally Posted by rekmeyata

Those set screws are there to help take up chain slack, in other words to set the chain tension. The tension should not be more or less than a half an inch when you push down on the chain in the center. Once you have the tension set correctly then you make sure the axle is aligned.

It took me a little while to find the CORRECT article on the internet that knew what they were talking about, because I can't explain how to set the screws without showing you, and I can't be there to do that. The internet had 2 pages of incorrect information as to what those screws were for. Without further ado please see:

Simple Ways to Adjust Horizontal Dropouts: 13 Steps (wikihow.com)