Submission grappling is a form of wrestling in which you control and submit an opponent without strikes. A coach teaches positions such as mount, side control and guard, the transitions between them, and the joint locks and chokes that end a match when the opponent taps out.
Grappling is practised both in the gi, a jacket and trousers you can grip, and no-gi, where you grip the body instead of clothing. The technique is close to Brazilian jiu-jitsu and MMA ground fighting, so people also come to grappling classes from those sports. The first thing you learn is to tap in time, which keeps locks and chokes safe.
A typical session has a warm-up, technique drilling with a partner and sparring, where a beginner starts calmly with more experienced training partners. Before choosing a club, ask whether classes are gi or no-gi, whether there is a separate beginners' group, how often you spar and whether the club competes in grappling events.