An Entombed Piece in Chess is a piece that is rendered useless because of it’s position on the chessboard being trapped by your pieces. No matter where the piece moves, it would simply be captured making it useless.
This is commonly seen with a Knight. The Knight can be trapped by just a couple pawns. No matter where the Knight moves, it would be captured by a pawn.
How To Prevent
You can prevent this by always being on the lookout for ways in which your opponent can trap your pieces stripping it of any and all mobility, rendering your piece useless. Inversely, always be on the lookout for ways in which you can trap an enemy piece making it useless and have zero influence on other pieces.
This can be a huge advantage as you may imagine, especially as the game gets closer to ending as each piece becomes more and more essential to be of use.
I hope this guide on what an Entombed Piece is in Chess. If you want to read more Chess Terminology terms and definitions, these are important to know.