This song is about marijuana. Paul McCartney cleared this up in his 1998 book Many Years From Now when he explained that it was not about a particular person, but his desire to smoke pot.
Session musicians played trumpets and sax. It was the first time horns were used in a Beatles song.
Earth, Wind & Fire recorded a funky new version for the 1977 movie Sgt. Pepper's Lonely Hearts Club Band. Beatles producer George Martin was in charge of the music, and the soundtrack was a success, but the movie, which starred Peter Frampton, The Bee Gees and Aerosmith, was a huge flop. Earth, Wind & Fire's version of this hit #9 in the US.
This didn't chart until 1976, when it was released as a single backed with "Helter Skelter."
John Lennon thought this was some of McCartney's best work.
In the '60s, Joe Pesci was an aspiring singer known as Joe Ritchie. He recorded a version of this that can be found on Rhino's "Golden Throat" Series. His version merits the "Stick to Acting" award. (thanks, Barry Kesten - Bellmore, United States)