To grapple means to seize someone and engage in close combat, usually holding them against yourself while you wrestle for control. Often, two grappling fighters, unbalanced by their efforts, will topple over and continue their struggle on the ground.
Grappling was a feature of medieval combat, however, and several weapons were developed with it in mind, particularly the poignard, the dirk and the ballock dagger, all knives designed to be slipped through gaps in armour and into reletively unprotected flesh.
Grappling in SCA
Grappling in Re-Enactment
Grappling does not typically occur in re-enactment combat unless it is specifically part of a scenario, or type of combat or the two combatants have agreed to allow it beforehand. Shows typically include grappling as part of the show.