A ‘blind steal’ is a raise before the flop with a fairly weak hand, with the goal of getting all opponents to fold and win the blinds uncontested. Usually this happens in late position, from players in the CO/BTN, when all players have folded to them.
In online 6-max games the blind stealing play is an important consideration as a way to improve your win rate because the blinds come around quicker with less players and players are capable of folding unlike live games where players will always defend their blinds against a small raise.