
They are using their feet and grips to stop you. Control their legs, get past their feet, and pin them.
Passing the guard is how you get from in front of their legs to a pin, and it scores 3 points. Against an open guard your opponent is using their feet and grips to block you, so your first job is to clear those grips and control their legs.
Guard passing comes in two broad styles: pressure passes like the knee slice and over-under, where you stay heavy and close, and speed passes like the toreando and leg drag, where you move around the legs. Whichever you use, finish the pass. Stopping half way lets them recover guard.