Attacked pieces
An attacked piece is a piece a unit of the other side can capture on this move, subject to the rules of chess (including pins that actually prevent the capture). In a game you do not get a banner that says “something is hanging.” You have to look.
The signal
After you look at the position, ask: which of my pieces are attacked, and which of the opponent’s pieces are attacked? Mark every such piece. The habit is completeness, not finding one flashy capture.
Why it is easy to miss in a game
Puzzle sites usually put you in a position where a capture or a tactic is the point. Over the board, attacks appear after quiet developing moves. A pawn push, a recapture, or a piece stepping onto a new diagonal can put a rook or queen under fire while you are still thinking about the opening.
If you only calculate when you already “feel” a tactic, you skip the boring scan that would have shown the attack.
What WayChess trains
You practise marking all attacks on your pieces, then all attacks on the opponent’s pieces, on many positions. Feedback tells you whether you missed a unit or marked a piece that is not actually attacked.
That scan is the first habit in the WayChess path. It is also the one with a guest try. See the other habits or how it works.