Guys, it's rude to do private jokes in public. Whatever this trio are laughing at, we're hoping they shared it with the poor photographer in the end. Here we have Joni Mitchell, Mimi Farina, and Herbie Hancock finding a great deal of humor in whatever it is that Mitchell is reading. Maybe it's fan mail? Or maybe it's reviews of Mitchell's latest album - the picture itself was taken at Berkeley Jazz Festival at the Greek Theatre in 1979, at a time that Mitchell was experimenting with releasing jazz records. It was a major departure from her usual style and wasn't universally well received.
Hancock, by this point already a pioneer of the post-bop sound, would later go on to fuse jazz with electronica and remains influential to this day. Farina had already more or less left the music industry by this point, choosing instead to focus on charity work.