A pin is a situation where a piece can't move (or shouldn't move) because it would expose a more valuable piece behind it on the same line. In an absolute pin the piece behind is the king, making the move illegal; in a relative pin moving is legal but costly. Only bishops, rooks, and queens can pin.
A pin is chess's way of freezing a piece in place. The pinned piece still stands on the board, still looks active — but it can't do its job, because a line-moving piece stares through it at something more valuable behind.
After 1.e4 e5 2.Nf3 Nc6 3.Bb5 — the Ruy Lopez — White's bishop on b5 pins the knight on c6 against the king on e8. The knight is not attacked by more pieces than defend it; it is simply stuck. It cannot legally move, because doing so would expose the king to check.
That is an absolute pin: king behind, movement illegal. A relative pin has a queen or rook behind instead — the pinned piece may legally move, but usually loses material if it does. Relative pins hide a trap, though: if the pinned piece can move with a big enough threat (a check, a mate threat), the pin evaporates. Assuming a relatively pinned piece is paralyzed is a classic way to walk into a zwischenzug.
A pinned piece is a compromised defender. It may still 'defend' a square on paper while being unable to actually recapture — countless combinations begin by noticing that a key defender is pinned and therefore fictional. Pins also invite the classic follow-up: pile up on the pinned piece. Attack it once more than it is defended and it falls, since it cannot run.
Pins pair naturally with other motifs: a pinned piece is effectively a loose piece for tactical purposes, and attacking it is a form of exploiting an overloaded defender.
Three standard remedies: break the line (block with a lesser piece, or move the valuable piece off it), challenge the pinning piece (the h6/g5 bishop-question is the everyday example), or generate counterplay big enough that the pin doesn't matter. What you should rarely do is ignore a pin and play on as if the pinned piece were a normal defender — that assumption is behind a large share of amateur blunders.
If pins are a recurring theme in your losses, your games will show it — and Chessdock will turn those exact positions into training material.