The song is about this guy named stan who is over obsessed with slim ( Eminem). In the song stan writes letters to slim but he never writes back. So stan goes through a mental break and puts his pregnant girlfriend in the trunk of his car. Once she is in the trunk of the car he starts driving 90 MPH down the road drunk with intent to drive off the bridge and commit suicide with his girlfriend in the trunk. He is also at this time recording a tape that he intends on sending to slim, but then he realizes that if he drives off the bridge he won’t be able to send the tape to him. He realizes this to late and drives over the bridge killing himself and his pregnant girlfriend.
Fortunately stan is just a made up character for the four verses of the song and never really did such atrocities.