En prise (French for 'in take', pronounced roughly 'on preez') describes a piece that can be captured for free or for inadequate compensation. Saying a piece is en prise is the traditional way of saying it hangs.
Chess kept a certain amount of French from its café days, and en prise is the piece of it you'll hear most often. A piece is en prise when it stands where the opponent can simply take it — either it is undefended, or its defenders are worth less than the piece itself.
In modern casual usage, en prise and hanging mean the same thing. If your bishop is attacked by a pawn and defended by nothing, it is en prise; it hangs; it is about to be free material. Older books lean on the French, streamers say 'hanging' — the concept is identical.
What makes the term more than trivia is that pieces are sometimes placed en prise on purpose. A piece offered as a sacrifice is en prise by design: yes, you may take it, and taking it is exactly what the attacker wants. The famous Greek Gift sacrifice puts a bishop en prise on h7; accepting it drags the king into a mating attack.
So 'the rook is en prise' states a fact about the board, not always an error by its owner. The critical question is whether capturing is safe — which is why automatic captures are one of the biggest sources of blunders. Free material that looks slightly too free deserves a long look — it may be a sacrifice, or the capturing piece may end up trapped far from home. Checking what happens after you take is precisely the kind of discipline a blunder-checking habit builds.
Your opponent pushes a pawn forward, attacking your queen, and in relieving the attack you notice their knight sits en prise on d5 — attacked by your bishop, defended by nothing. If nothing tactical is hiding behind the capture, that knight is simply yours. Games at every amateur level contain several such moments; the players who spot them consistently win the material battle without any brilliancies.
Training that awareness on your own games is what Chessdock is built for: it finds the moments you missed — pieces you left en prise and pieces you failed to take — and turns them into puzzles from your own play.