A "plump" person is a little bit round and fat, though not too fat.
You can call someone "plump" if they are this shape. Or if someone is quite fat, you can call them "plump" as a way of being nice instead of calling them "fat" or "obese". However, "plump" isn't such a polite expression that you can freely say it directly toward a fat person.
Food that is round and soft can also be described as "plump":
I'm cooking a plump, juicy chicken for tonight.
When you're describing food, "plump" is usually good.