In Texas Hold 'em, having 2 and 7 off suits is rated the worst hand. This is because they're the two lowest cards you can have, and they can't create a straight (there are five cards between 2 and 7).
A 2-7 offsuit hand is the worst hand to start with in Texas Hold 'Em poker because there are so few good options available to you: you have no straight draw, no flush draw, and even if you wind up with a pair of 7s or a pair of 2s, you're unlikely to have the best hand.