A Blind steal means that a player try to attack the small and the big blind with the goal to steal them. Especially if a player sit in the late position, it make sense to steal the blinds. If the player who are sitting in the blinds are very tight, you can raise with almost all hands to steal their blinds. If they players who are sitting in the blinds defend them very often, then it make of course sense to steal not to much and to choose the hands carefully.
A good blind steal range is a must have for every good poker player.