Trending topic means popular topic
Trending topics are a series of keywords that are most mentioned on Twitter at that time. Trending topics are a way to see where a lot is talked about on Twitter, and each country also has its own trending topics. Nowadays, there is a lot of talk about trending topics outside of Twitter, but the idea remains the same: these are subjects that are often talked about in any way whatsoever. On Twimmer is a nice overview to see what the trending topics are.