It's being reported as a banana thrown at Gareth Bale, but that does not really make much sense. Not that racism makes sense, but usually the racist European soccer fan reserves his banana-related taunts for black players. As you see in the video, the referee clearly picks up a banana thrown on the pitch just in front of Gareth Bale (who, by the way scored again today), but he seems rather unconcerned about it. What would make more sense would be to consider Arsenal's relationship with Bale's teammate, Emmanuel Adebayor.
In 2009, after a bitter end to his time with Arsenal, Adebayor scored a goal against his old team and ran the entire length of the pitch to slide and celebrate in front of Arsenal fans. The fans showered him with vitriol and, likely, bananas.
Again, it's unclear from the video whether the banana was thrown at Adebayor specifically, but here are the things we know: Arsenal fans do not like Adebayor and someone brought a banana to a soccer match he was playing in and threw it on the pitch.
Update: It's been pointed out that some say Bale looks like a monkey. I never thought it until it was mentioned below but I can see it now. That makes more sense than overt racism, but there is obviously a history there.