A few years ago we had a run of animated films where the hero was a "bad guy". I don't mean merely a "jerk with a heart of gold" like Lightning McQueen in Cars. I mean films that sold themselves on the idea of "villain has to save the day". Notable releases to use this concept during the period I'm thinking of include Despicable Me, Megamind, and this film. They certainly weren't the first to do it - Shrek is an obvious precursor for instance - but they all came out in a period of a few years, making the trend quite noticeable.
Disney apparently first came up with the concept for this film some twenty years before its eventual release. I think the long production delay probably worked to its advantage. For one thing, it gave computer animation time to reach the point it needed to be at for the movie to really pop off the screen, and - perhaps even more importantly - it gave video games the time to become such a well-established and ubiquitous part of culture that the audience would instantly grasp many of the in-universe concepts. Having to stop and explain the context - which in say 1990 I am pretty sure they would have needed to do - would rob the film of momentum.
All that blathering aside, what's the movie about and is it any good? Well as the preamble suggests, it is about a "bad guy" who has to save the day. Specifically it is about Ralph, who is the villain in a Donkey Kong-esque arcade game. Tired of being ostracised by the other characters from his game, Ralph decides to prove that he could be a hero. Of course, that's not going to be as easy as he expects.
As for whether it is any good: Yes, yes it is. It has plenty of funny sequences, likable characters, nice animation, and a great voice cast. Really the only reason not to see it would be if you're one of those "animation is for kids" stick in the muds, and really, that's your loss if you are.