A claim or (concession) is a statement that your side will win a certain number of tricks without actually playing them out.

A claim should be accompanied by a clear statement of your ‘line of play’, i.e. the order in which remaining cards will be played. (You can’t just say, “The rest are mine” even if it is obvious.)

It is done as a courtesy to the opposition, and the Laws encourage it.  Indeed, the etiquette Law expressly discourages “prolonging play unnecessarily for the purpose of disconcerting an opponent”.  There is no benefit in waiting for the opposition to make discards when they can never win another trick.

Play ceases immediately the claim is made, unless all four players agree to continue.  If there is any doubt or the line of play is not stated, call the Director.  Any doubt about a claim is resolved in favour of the opposition.

Some players don’t like claims.  They think they are being cheated.  This is hardly ever the case, although occasionally a player may make an incorrect claim in all honesty.  You should always just politely check that the claimer is correct.  If you disagree with the claim you should call the Director.  

Some recommendations

Wait until all trumps have been played before claiming.

Don’t claim until you have only 3 or 4 cards remaining in your hand.

Lay your cards on the table one-by-one in the order that you intend to play them, stating what you are going to do.