The trollbox is relatively new to Everybody Edits. It is just what it sounds like - a small box in a world that can be trolled (you are supposed to troll in it).
This is a way to troll without disturbing to other players.
Alternatively, it can be "trolled" (filled) with BETA bricks.
The first appearance of a trollbox (a BETA trollbox) was in Everybody Edits user jDodson99's level "BETA BOSS AND ART," which is now, as of this writing, the first Delta Duo level (INFERNO)