This song is the third most played one in my iTunes. It came out when I was in college and a couple years after I had seen them. It sort of revived my flagging interest in them.
The song deals with profound loss -- of what we don't know. Here are the usual "theories" link
I gave Fleetwood Mac another listen after seeing Dave Grohl's "Sound City"; he interviewed Mick Fleetwood and Stevie Nicks about how that studio brought that band together then.