A premove is an online feature that lets you enter your next move while your opponent is thinking, so it executes instantly when your turn comes.
During your opponent's turn you drag or click your intended reply, and the site queues it. The moment your opponent moves, your premove is played automatically if it is still legal, costing either nothing or a tenth of a second depending on the site. If it has become illegal, it is simply cancelled.
Premoves are the backbone of bullet and of blitz time scrambles. Obvious recaptures, forced king moves and long planned pawn pushes can all be queued, letting you play whole stretches of the game at essentially zero cost on the clock.
The danger is that a premove executes against any legal reply, not just the one you expected. Premoving a recapture into a zwischenzug, or a king move into a mating net, hands your opponent a free tactic. Queue a move only when it stays sensible against everything.
It depends on the site: some charge one tenth of a second per premove, while on Lichess a premove costs nothing. Either way it is far faster than any human reaction, which is why premoving matters so much in bullet.
Whenever your opponent has a forcing option that changes the picture, such as a check, a capture or a promotion. If any legal reply would make your queued move bad, wait and look first.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.