Through the course of her current tour, Taylor Swift has been bringing out various famous musicians and singing one of their songs with them.
On this night, Swift's 4th show in a run in Los Angeles, sghe brought out Beck and Annie Clark (St. Vincent) and they all performed Beck's current single, "Dreams." Beck and Swift sang, Clark played guitar - including a bit of a flashy solo in the middle. I don't know who the rest of the band was, or if it was played to a tape, or what.
Beck has been flirting with the mainstream for a few years now, and this seems to be the pinnacle.
(The night before, also at Staples, Beck took his family to see Taylor's show.)