These are judo-style competitions it which you have to overcome an opponent to win. Think about sumo wrestling in the ground. You begin in the centre of a circle and have to push your opponent out without rising to your feet. This is groundwork. You must pin your opponent or force them to submit without rising to your feet.
Higher grades can use strangles.
Randori is a contest of throwing ability. The rules are much the same as for judo and to win you throw, pin or force a submission. The morning courses alone make it worthwhile. The competition stuff is all appropriate to each level. On Sunday afternoon you can watch the high grades do their stuff and the social side is very strong.