Apollo Creed has been killed in the ring by Ivan Drago, and Rocky needs to get his head straight. He wants to fight Drago to avenge Apollo and prove that he can still fight, but he's not sure he can win. So, Rocky goes out driving and ponders the first three movies that brought him to where he is, how much Apollo meant to him, and how he must fight Drago.
This is really an outstanding montage. In fact, it's probably the ultimate montage: forget shots of scenes with time gaps between them, this montage has shots of scenes that didn't even happen in this movie! Not to mention you have Rocky zipping along in his Farrari, a cinematic masterpiece.