A slab is a piece of rock that is less than vertical. I don't normally climb slabs preferring pieces of rock that are more than vertical, these suit the way I climb, without feet. Just by the road leading down to the college is a particularly fine slab of rock that I have tried to climb many times over the past 17  years. The problem is that it doesn't have a great deal of holds on it. You have to put your foot  into a small depression in the rock and stand up hoping it doesn't slip. The problem is that friction is related to the normal force so until your weight is on the foot  the friction is very little. It's difficult to trust the foothold until you are actually standing on it. Dynamic friction is also less that static so once your foot starts sliding keeps sliding. The trick is to put weight on the foot quickly but smoothly and that takes confidence.