Your opponents are entitled to know all your partnership agreements, just as you are entitled to know theirs. This is one of the reasons you must fill out a convention card before the start of each game. This is also why you alert non-Standard bids and why you must give a full explanation when an opponent asks the meaning of your partner’s bid.
The time to ask for an explanation of an opponent’s bid is when it is your turn to call and before you make your call. The partner of the bidder gives the explanation, and the person who made the bid must not do or say anything. If the partner of the person who asked a question wants a further explanation, they must also wait until it is their turn to call.
The proper form is to ask something like “What is your agreement regarding your partner’s 2 club call?”, not “How do you take that bid?”. How they take that bid may be based on their own hand and they do not need to disclose that.
The explanation should include all relevant information regarding your partnership agreement. Simply stating the name of a convention, like “Cappelletti” is not adequate. You should say something like “Relay to two diamonds, showing an unspecified 6-card suit” or “Shows spades and an unspecified minor, likely 5-5”. You do not need to say anything about inferences you draw from general bridge knowledge or from your own hand. However, if your partner has a tendency to have only a 5-card suit, you should say so even if you have never openly discussed it. If you have no agreement, simply say “No agreement” or “undiscussed” and the discussion should end there. Don’t embroider: “I think we’re playing Cappelletti”. Follow-up questions like “But what do you think it means?” are improper and you should not answer. Meanwhile, partner should be studying their cards and mentally counting backward from 100 in French to avoid making eye contact and giving you any hints.
Sometimes mistakes happen. Partner fails to alert your conventional bid, or alerts a bid you thought was natural. The alert or failure to alert is unauthorized information for you. You must not do or say anything to indicate you disagree, and you must continue to bid as though partner shared your understanding of your bid. Imagine you are playing online and cannot see partner’s alert and explanation. Likewise, your startled reaction is unauthorized information for partner. They must also continue to bid as though your bid meant what they thought it did. Once the auction ends and before the opening lead is faced is the only time you can look at your convention card and find out who was right. If your bid was wrong and partner’s explanation agreed with what is on your convention card, you do not need to say anything. In fact, it’s best if you don’t give the opponents any hint that you are in trouble, lest it helps them savage you during the play. However, if partner’s explanation is wrong you must correct it at your first legal opportunity. If you are declarer or dummy that is as soon as the auction has ended and before play starts, but for defenders that is at the end of play. (To do so earlier would give your partner unauthorized information regarding your hand.) Call the Director, and then tell the opponents they may have been given the wrong information.
A final point: you may not ask a question if your sole purpose is to benefit your partner. Even if you are playing with a novice and you’re pretty sure they do not understand what an opponent’s bid means, it is not your place to ask for them.