Graveyard Mechanics refers to gameplay based on the things that are normally no longer in play, such as the discard pile. For example, when a creature in Magic: The Gathering dies, it goes to the discard pile, called the graveyard. Normally, that's the end of it. However, there are some cards that can do things like "raise the dead" and bring it back. Then there are other cards that provide some kind of benefit based on the number of creatures in the graveyard. When designing a deck around this mechanic, the goal becomes actually trying to get more things in your discard pile, as it only plays to your benefit.

Recently, the Pokemon franchise has started to introduce graveyard mechanics in its new games. There is a move called "Revival Blessing" that can revive a fainted Pokemon in the player's party. Up until now, that wasn't possible unless an item was used, which meant it wasn't at all possible in tournament-style matches. Another move called "Last Respects" gets stronger based on how many Pokemon in the player's party are fainted. This means that sending out the Pokemon last or after a few others have been defeated makes it extremely powerful. Finally, there is an ability that gives a stat boost to a certain Pokemon based on how many other Pokemon in the player's party have fainted.

In other games, graveyard mechanics involve the use of dead enemies as a resource. In Diablo 2, for example, the Necromancer character relies almost entirely on "graveyard mechanics". He can raise the corpses of fallen enemies as his personal skeleton soldiers, or he can cause them to explode or restore his health.

I don't know if I would call enemies that come back from the dead or use the dead "graveyard mechanics", as they are technically outside of the control of the player. Halo's Flood, for example, use graveyard mechanics, but the player can't do anything about it. The same goes for any enemy in any game that keeps rising from the dead.

Also, I wouldn't call standard "revivals" to be examples of graveyard mechanics, either. This refers to the ability to revive a downed teammate within a short period of time before they are dead dead. I see it as more of a standard gameplay mechanic involving positioning, timing, and opportunity.