Sealing the hinge gap means to take a strip of covering about 3/4" - 1" wide and as long as the surface. Fold the strip in half WIDTH WISE. Flex the control surface as far as it will go and insert the folded strip in the opening. Using a TRIM IRON, iron 1/2 of the strip to the control surface and the other half to the main part of the structure (trailing edge of the main wing ).

The purpose of sealing the gap is to prevent air from getting in the gap and causing flutter.

FWIW - I have found that for regular sport flying, unless the gap is really large (over 3/32" ) you don't need to seal it. If the gap is large OR you will be doing ANY high speed flying, you should seal it.