A knight lands on d5 in the middle of the board, looking magnificent, attacked by a pawn on e6 and defended by nothing.
7 
2 
a 
d f 
g That is en prise. The phrase is French, it turns up in every annotated game you will ever read, and it means a piece is sitting where it can be taken for free. Attacked, and not defended, or defended by something worth more than the attacker.
Attacked is not the same as en prise
A knight on d5 attacked by a pawn and defended by a bishop on c4 is fine: pawn takes knight, bishop takes pawn, and you have swapped three points for three plus a pawn back. Move that bishop to d3, as in the diagram, and the same knight is a gift.
So the test has two halves. Who attacks the square, and who defends it, and what are they all worth. Beginners check the first half and forget the second, which is why the same knight can be safe on Tuesday and lost on Wednesday without moving.
The check that saves you a piece a game
Before you touch a piece, run two questions.
What does his last move attack? Every move changes something. If his bishop just went to b5, look down the whole b5 to e8 diagonal before you think about your own plans.
What of mine is undefended right now? Not attacked: undefended. Undefended pieces are what tactics feed on, because a fork, a pin or a discovered attack only wins something if the something is loose at the end of it.
Ten seconds, every move. It feels slow for a week and then it becomes the way you look at a board.
When it is deliberate
Sometimes a piece stands en prise on purpose, and taking it loses. A knight on d5 that cannot be captured because the pawn on e6 is pinned against the king is not really hanging, it just looks it. A bishop offered on b5 that opens a mating net when taken is a sacrifice, not a blunder.
Tell them apart by asking what happens after the capture. If the answer is nothing, it was a free piece and you should take it. A free piece is a free piece.
The habit is worth building on purpose: hanging piece puzzles are the cheapest rating points in chess, and the routine version of the two questions above is in how to stop blundering. Then count what you won, in the plain arithmetic of material.