When you take a hit of weed and hold it in so long that nothing blows out when you exhale. While difficult depending on the size of your hit, it will lead to a much higher high. Graveyarding is most frequently done by those who are running short and are trying to conserve until they can pick up again. Caution - can lead to uncontrollable coughing.
"Mark doesn't get paid until Friday and only has a small stash left. He will have to graveyard all his hits to make sure he can get high until payday."