As they said above for the meaning.
the release of the MOVIE Titanic,
Movie is implied. The rest is description of what sort of movie.
massive box-office hit (movie)
So "massive box-office" is used as an adjective to describe the type of movie.
Strictly, "massive" means "has large mass", that is "is heavy here on earth", but people use it to mean large. A lead weight tends to be massive for even modest size.