The /dev/null is a tool added by OpenBlocks. This tool is used to discard all of a single item type when they are picked up. It has a single inventory slot, which can be accessed through sneak-using it when not targeting anything. The item inserted is the "filter" item. Once the stack in that slot is full, all exact items will be immediately discarded on pick-up.
If the /dev/null has a block in its slot, it can be used to place the block.
/dev/nulls can be placed inside of another /dev/null. Doing this 4 or 5 times will give the player the advancement "Stack Overflow".