When someone is prone, they have adopted a position laying on the ground. This occurs naturally when adventurers are either seated or sprawled on the ground by their own choosing or placed in that position by another. Several class abilities can force someone prone, as well as wielding the Protean relic, the Earthshaker.
Being prone makes adventurer targets vulnerable to a host of offensive abilities, such as Tekura Backbreaker, Trample in Dragoncraft and Riding, and Weaponry Behead. Simple STANDing relieves this particular susceptibility.