It indicates the stage of play when most of the pieces have been exchanged, and the board is relatively clear.
Material is approximately equal for both sides, and the opposing Kings can become aggressive.
Often, there is a critical point where the outome will be decided by one key play. East and North cannot obtain the lead, have made discards on a previous lead of hearts, and they both hold all of the low diamonds. There are four hearts remaining , as well as two top spades. West's clubs are safe, but are winners. Finally, South is on lead.