Trapped piece
A trapped piece cannot reach safety: every square it might retreat to is covered, blocked or ruinous, so once it is attacked the owner will lose it for little or nothing.
Being attacked and being trapped are different things. A piece can sit calmly for many moves, and then one quiet pawn move closes its last exit. What decides it is the list of retreat squares: if each one is covered by the enemy, occupied by your own pieces, or leads to a losing exchange, the piece has no future.
The usual victims are bishops that grab a flank pawn on a7 or h7, knights that jump to the edge of the board, and queens that wander into enemy territory for a pawn. Rooks can be trapped too, usually on an edge file by minor pieces.
The recipe for winning a trapped piece has two ingredients. First cut off the last exit, often with a pawn or a quiet move. Then attack the piece with something cheaper, such as a pawn, a king or a minor piece. If the owner can only give it up for a pawn, you have won material.
Self-defense is mostly counting. Before capturing a pawn on the far edge, count the retreat squares, think about how the opponent could close them, and keep a way back. If a piece is already doomed, look for the most you can get for it.
In the diagram it is Black's move. After b6 the bishop on a7 has only two squares: b8, which the king on c8 covers, and b6, a pawn that c7 protects. Next Kb7 attacks the bishop, and White can recover at most the b6 pawn.
Also worth reading is Piece domination. Piece domination is a position in which an enemy piece cannot move without being captured, because your pieces control every square it could go to, so it is stuck and out of play.
A neighboring idea is Poisoned pawn. A poisoned pawn is a pawn that can be captured, but whose capture lets the opponent punish the greedy player with a tactic that costs more than the pawn is worth.
Compare it with Domination. Domination is a plan in which your pieces and pawns watch every exit of one enemy piece, so that it can neither move safely nor contribute, and it may even be lost.
Knowing "Trapped piece" helps most once you can spot it in a real game. A first lesson with George Li starts from your own games and finds the tactics you keep missing.
Related terms
A hanging piece is a piece that is attacked by the opponent and has no defender, so it can be captured for free unless its owner moves it, defends it or makes a bigger threat.
A loose piece is a piece that no friendly piece or pawn protects, which makes it a target even when nothing attacks it yet and gives the opponent a chance for a double attack.
A fork is a move that leaves one of your pieces threatening two enemy pieces together. Because the opponent gets only one turn to respond, something usually falls.
A pin ties a piece to its square: a bishop, rook or queen attacks it, and stepping aside would expose a more valuable piece standing behind it on the same line.
Guides that go further
What to decide first, the questions to ask, what a first lesson should show and the red flags to avoid.
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A game review, a read on your level and a written study plan from George Li.
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A first lesson starts with your games: George reviews them and builds a study plan around what he finds.