The ability of a film to entertain its audience is the product of a number of factors.
Film, like paintings and novels, is a form of art. How one responds to that art is entirely dependent upon the personal tastes of each individual viewing or reading the item in question. Not surprisingly, whether a film, novel, or painting is "good" is entirely a subjective matter. We each have our own opinion of what we like and dislike.
In the specific case of film, which more than the other forms of art can manipulate the emotions of the audience, the people making the film strive to accomplish goals consistent with the underlying story line. In short, what makes a film entertaining is how it captures the audience's attention. If the film is a comedy, it's entertainment value can be measured in terms of the how much the audience laughs. If the film is suspenseful, the filmmakers hope that it captures the attention of the audience and makes it nervous, frightened or angry. So-called slasher films respond to the desire on the part of many film-goers for violence and bloodshed, usually at the expense of people who "had it coming."
What makes a film entertaining is whether it accomplishes its objective. If audience members exit the theater humming or singing a song from a musical, that film probably succeeded at entertaining. If they exit the theater discussing the identity of a murderer, the "who dunnit" succeeded in capturing and holding the audience's attention. If you liked the film, then it was entertaining.