It's a Mystery box.
What Can They Yield?
Inside the boxes, a player can receive any type of item, ranging from ashes to the coveted Christmas cracker. The chances for the Mystery Box to rollout a rare item are very slim, resulting in tons of worthless items into the game. This is another reason why the players decide to just use the box as a monetary item.
Soulsplit developer Jayarrowz released a list detailing all of the available items in the Mystery box in this forum topic, with the odds of receiving each item based on the number of items shown; that is, the less items there are, the less chance you will receive that item from a Mystery box.
There are three ways to obtain a Mystery Box: