Definition of theft
Preflop steal, is a raise from the player in cutoff or button position. When one of these players raises, they are not just called a preflop raise, but a steal, since they are in a steal position. If a player adjusts and decides to 3-bet the stealing player, he is said to be restal (re-stealing, as it were). Stealing blinds in poker is a very common occurrence. Blinds are what drive every poker hand, otherwise players would just wait for a pair of aces to play. In tournament play, this is even more true.
To win tournaments, you have to steal blinds
In cash games, you can afford to be patient and wait for your hands, especially if you're playing full ring tables with 9 or 10 players. But when you play tournaments, your success will depend on your ability to steal blinds. You can't just wait to get good hands. Especially since at some stages of the tournament, the blinds will be a big part of your stack. It is necessary to steal blinds when you have 7-8 big blinds and the starting pot is about 3 big blinds with antes.