There are some cards, that have activated abilities, that a player can activate when the card is in the graveyard. For example, the ability of Stitchwing Skaab to return it from the graveyard to the battlefield.
However, most of the other card abilities, a player can't activate from the graveyard. For example, Drogskol Cavalry's ability to put a spirit token to the battlefield can not be activated when it's in the graveyard.
So my question is, which rule of the game allowing a player to activate some abilities from the graveyard?