From Bulbapedia, the community-driven Pokémon encyclopedia.
The Tight Belt is a hold item in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team and Pokémon Mystery Dungeon: Explorers of Time and Explorers of Darkness. When held by the leader, it prevents the belly from emptying unless a linked move is used, or if a non-Ghost-type Pokémon uses a Mobile Orb to walk through walls, each of which will decrease the belly by one point each time used/each turn in the wall.
The Tight Belt is often considered the most valuable hold item in the games, however, they are very rare and difficult to obtain. It can only be found in the Far-off Sea and Purity Forest dungeons in the first Mystery Dungeon games, and can only be obtained in the dungeon Zero Isle in the sequel.