Typically, "stealing" is when you have a hand that is not good enough to raise, but you raise anyway, based on your position, your table image, your opponents' table image, how often the players in the blinds are defending them, and ultimately hoping that your opponents don't have a good enough hand to call your raise.
Stealing from the UTG works best when the other players at the table are afraid of you and/or very conservative. They may feel that you don't have a particularly good hand, but they feel that you can outplay them after the flop. Of course, this is the advantage that poker professionals have over their opponents.
I first noticed this action at the WSOP. The tournament was in the middle phase and a poker pro, Erik Seidel, was in UTG position. He had just 15 big blinds. Erik raised 3x, representing a particularly strong hand, and that was enough to get all his opponents to fold.
In the next orbit, again in the same place, he did the same thing again, and again everyone dropped out, not wanting to mess with him. At the time I thought he was just getting a very good hand, but then I found out that this is just the action that poker pros do to combat the constant increase in blinds.
Eric's image is that of a smart, conservative and closed player. His opponents at the table were also conservative and waiting for good hands. All these things combined made it very easy for him to steal the blinds from this seemingly very unfavourable position.
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