Ring gap = space between the rings where it meets the pin.
Space should be .010 or there abouts. Formula is .003 to .005 per inch of bore.
To check, install ring [s] on piston, slide piston into jug...not far... to compress ring. By turning the piston you can expose the area where the pin is. You'll notice a gap where the ring comes together at the pin which you want to measure with a feeler gauge. If it's not at least .010, slide piston back out, remove ring and gently file the split where it meets the pin.
Repeat above until gap is correct.
This is done to allow room for expansion once things heat up. If you do not than you are running the chance of a piece of your ring breaking off.
I know of at least one member who just throws his kits on without even looking at the ring gap. So far so good but one of these days..remember I said that Reynold. :)