Rules & Etiquette
When it is your turn to bid, you may ask either of your opponents to explain a bid that their partner has made. You can do this whether their bid is conventional (alerted) or natural. You may not ask one of your opponents what his own bid means.
Do not ask during the bidding unless you think it will affect whether or what you bid - you may help to clear up a misunderstanding between your opponents!
If you know that it will not affect your bid, but you still want to know what the bid means, you may ask once the bidding has been concluded and before the first card has been led. If it's your partner's turn to lead to the first trick, he should ask if you have any questions about the bidding before facing his card.