Dan Harrington coined this phrase to describe the fact that the first player in a tournament to open a hand with a raise or all-in has an advantage; all players after him must have significantly better hands to call his raise than he needed to make it. Therefore, they will be forced to fold cards which would otherwise be playable.
This is particularly important late in the tournament or when one has a stack that is small in proportion to the blinds, since a player then seldom has the time to wait for a good hand and so can use this advantage by being the first to go all-in on a hand. On the one hand, he puts pressure on the other players who often have vulnerable stacks; on the other hand, he uses the fact that the players directly after him cannot call with a good hand since there will still be players acting after them that could have a very good hand. They will usually fold a lot of hands that they might otherwise have raised.
This can be seen as a practical application of Sklansky's gap concept, which says that it always takes a better hand to call a bet than to make it in the first place.