Broadly, I'd say it's a question that contains premises that are are either debatable or deliberately leading. (This is what "begging the question" actually means). It also refers to questions that force a candidate to waste valuable time countering a charge that the moderator wants to make, such as, "Mr. Trump, many say that you're despicable to women. What changes do you plan on making to improve that?" or "Mr. Cruz, can you cite a single legal decision that would allow a Canadian citizen such as yourself to run for President?" There are others that are simply silly, such as "how does magnetism work?" that have nothing to do with politics and everything to do with trying to manipulate the candidate into embarrassing himself or herself.
Sometimes it isn't quite so cut-and-dried. For example, I'd have to say, "Yeb!, what in God's green earth are you doing at that podium?" is a fairly legitimate question...