Part of the story of this song is absolutely the video. Singer and principal songwriter Karen O cries during the video, for a song written about her relationship with Angus Andrew from the Liars. He was supposed to come to the shoot, and was quite late - so when he showed up, it was an emotional moment.
But really, like all of these songs on this blog, it's about the song. And this one is a mostly vague, emotional tour de force, with the strong anguish and outpouring declaration of an unequaled love serving as the chorus.
The song's title is a mystery. Some thing it's an abbreviation for "My Angus Please Stay", and others think it's a allusion to the places they visited on tour. I don't think it matters. I think it's a great song.