The goal on offense is to score. Offensive rating is a fancy way of measuring how many points an average possession is worth for a particular team. The math involved is not terribly complicated when you dig down into it. Outside of free throws it is quite simple; field goal attempts plus turnovers minus offensive rebounds. Take that number, add in a tiny bit of math for free throws (.44 or .438 depending on where), and you have relatively accurate measure of the possessions a team had during a game.
Offensive rating is a measure of efficiency. There are a wide variety of paths to an efficient offense in the NBA; fast or slow, spacing or isolation, ball movement or player movement or both.