The information in the article below contains spoilers for the BFDI series. Please watch the episode(s) in question if you haven't seen them already.
The Mangosteen is the prize to join TPOT 1. Two calls it a purple tomato, which is most likely a reference to Reveal Novum, where the prize the debuter gets is a yellow tomato. When the debut elimination ends, Winner ends up winning and "eats" it.