It contains items.
Loot Sacks are what appear when a player drops an item on the ground. Loot sacks will also de-spawn after a set period of time (about 2-3 minutes of real time). Loot sacks are also dropped when a zombie is killed. Loot sacks have a VERY small render distance and can potentially be used to hide your valuable items. However this is not recommended, as they may disappear forever due to decay.
Loot Sacks act as any other container. To open a loot sack, a player presses "E" when aiming the crosshair at a loot sack. After a while the loot sack will despawn.