What do you guys understand under Cardiac Rehab?

So I've read that quite a lot of people go to cardiac rehab after a few weeks. What exactly do you do there? For context: I'm 23 and I had my Aortic Valve repaired last week tuesday and didn't have big issues with walking around and taking stairs. My doctors haven't recommended any rehab programms to me because it's just "walking around with a bunch of old people".