A loose opponent raises from early position and gets called by another player. The caller probably has a marginal hand or they would have re-raised the initial loose player.
The play is now on you and you re-raise knowing that the initial raiser is loose and could have a wide range of hands and that the caller must have a marginal hand or he would have re-raised himself. A large amount of the time both players will fold and you will win a large pot without seeing the flop.
The move is most effective when you are in position on both players, making it even more difficult for either of them to call your raise.