A buried item is an item that is found inside the walls of a dungeon. There are 4 ways to obtain buried items: you can break through walls with HM06 Rock Smash, use a One-Room Orb, use a ghost-type Pokémon (such as Shuppet or Gengar) or dig paths through walls with the IQ skill Supermobile. Buried items are very rare and are usually valuble items such as rare TMs or vitamins which raise your stats (Protein, Iron, Calcium, Zinc, etc.).
In Blue Rescue team, a sound will trigger when destroying walls near a Buried Item. (Only tested in Buried Relic)