A normal matchlock as you squeeze the trigger it lowers the match, on the snap lock it works like when a percussion hammer fires - snaps the match down onto the pan instead of lowering it in accordance to your trigger pull.

This is a normal matchlock https://m.youtube.com/watch?v=r6YZmkxY7JA

The snap version uses a spring. Apparently it has a tendency to put out the match (but still fire the gun), so was less popular with soldiers.