A loose piece is a piece not defended by any of its own pieces or pawns. It may be perfectly safe right now — but loose pieces are the raw material of tactics, which is why the classic maxim says LPDO: Loose Pieces Drop Off.
The phrase comes from grandmaster John Nunn, and it compresses a deep practical truth into four words: pieces that stand undefended tend, sooner or later, to fall off the board.
A loose piece is not necessarily in danger. Your knight on b6 might be untouchable at this moment. But it is undefended, and that makes it a lever for your opponent: any fork, pin, or double attack that includes the knight as one of its targets suddenly works, because the knight can be taken for free at the end of the sequence.
Look closely at almost any winning tactic and you will find a loose piece at the bottom of it. A queen check that also attacks an undefended bishop wins the bishop. The same check with the bishop defended wins nothing. Tactics are rarely conjured out of thin air — they exploit targets that were already there. No loose pieces, far fewer tactics against you.
This is also the fastest way to find tactics for yourself: scan for your opponent's loose pieces first, then look for a move that attacks two things at once.
The distinction matters: a loose piece is undefended; a hanging piece is undefended and attacked — it can be captured right now. Loose is the warning light; hanging is the crash. Strong players react at the warning-light stage, either defending the piece, moving it somewhere safer, or at minimum registering it as a permanent risk factor in their calculations.
Once per move, sweep the board and note every undefended piece — yours and your opponent's. It takes a few seconds and it transforms your threat awareness: most combinations that would have surprised you become visible one move earlier.
When you review your games, you will find that a striking share of your losses trace back to a piece that stood loose for several moves before it dropped. Chessdock surfaces those recurring patterns from your own games automatically, so you can train the scan on positions where it actually failed you.