Agreements, alerts etc
Pairs playing together are allowed to have artificial uses for bids in agreed situations. These are sometimes known as "conventions" and many are named after the person who invented or popularised them. They are played around the world, sometimes in different forms or variations.
The partner of the player who makes the artificial bid must immediately reach across and circle the bid. This is known as an "alert". It warns the opposition that this bid is not what you might think it is !
Note : A very strong 2C opening bid, although artificial, is not alerted as it is played this way as standard.
A 2C response to 1NT should be alerted IF it is natural, because most bridge players in the world use it artificially as a convention called 'Stayman', asking partner if they have a major suit.
4C or 4NT, asking for aces, does not have to be alerted, nor the responses. Observe the auction and ask at the end of the auction if you want an explanation.
Note: Bidding the opposition suit is usually some kind of agreement, but usually not alerted, it is considered "self-alerting" unless it has a very funny meaning. You can ask about these bids.
The opposition players are allowed to ask about the alerted bid BUT only when it is their turn to bid (or play a card after the auction). You can ask about any bid, but you have to be careful not to give a hint to partner why you are asking. I recommend that you ALWAYS ask about alerted bids, and NEVER ask about unalerted bids until after the auction. The correct way to ask is "Can you please tell me about this bid ?" If you are a Defender then you should ask Declarer to review the entire auction (so as not to hint at the bid you are interested in). You can ask for this review when on opening lead, or after your partner leads face down.
The question is asked of the partner of the player who made the bid being questioned. You don't ask the bidder becasue they will just tell you what they intended to bid ! When you ask the partner they must describe the pair's agreement. If they are unsure they must not guess, but say "undiscussed" or "we have no agreement". If they are wrong, their partner must keep quiet, AND pretend that they did not hear the explanation !
An answer should state the length and strength of the bid. That is, how many cards it shows, and the high-card point range e.g. "five or more hearts, 10-12 points". Naming the convention is not enough.
Firstly, if you answer a question and realise that you are wrong, you should immediately correct the explantion (and call the Director) even when it is not your turn. If you become Declarer and you think your partner has given an incorrect explanation then you must speak up (and call the Director). If you become the Defenders and your partner has given an incorrect explanation then you must not say anything until the end of play, and then you must speak up (and call the Director).
The Director has the power to change the result if necessary.
Failing to alert is misinformation and may result in a change of result, and possibly a further penalty !
MIS-BIDS and PSYCH BIDS
It is legal to make a bid that doesn't actually match your partnership agreement. You may decide to stretch a bit higher, or maybe back off and underbid. Or you may have just forgotten your agreement. As long as your partner has no indication that you are doing this, and bids according to your agreements, then the auction is legal and the result will stand. You aren't promising the opposition that you will have exactly what is explained.
If you make an incorrect 'unintended' bid you may be able to correct it, call the Director.
Some players will make a bid that is totally different to the agreed system. This is legal, and is known as a 'psych' bid. BUT these bids are only allowed once or twice in a session, and should be reported to the Director.