Junk Blocks are one of the obstacles found in Shovel Knight Pocket Dungeon, specifically in it's VS Mode.
By destroying chains of enemies and blocks, it will send gems into a sprite of the Junk Block that acts as an icon in the upper part of the screen. After enough gems have been consumed, the icon will indicate that a row of Junk Blocks/multiple Junk Blocks clumped together will descend shortly afterwards in the opponent's board. While on your board, it has one HP, same as the Dirt Block, so it's easy to get rid off.