To steal is take something that isn't yours: thieves steal. But a bargain that's so good it's almost like you're getting away with robbery is also called a steal.
Stealing is what thieves do: taking things from other people. A bank robber steals money. A mugger steals purses and wallets. People also use steal in other ways. If someone wastes your time, you might say they're stealing your time. A store having a sale might say, "Our prices are a steal!" In baseball, a runner can steal a base. When you see steal, think "take."