This one's going to be straightforward, Shmoopers. The main conflict of the film centers on a war between an evil empire and some underdog rebels. The story also takes place among the stars, a.k.a. outer space. Put them together, and you get Star Wars.
Sure, it's simple but that doesn't mean it's not a good title. It's catchy, memorable, and makes one heck of an eye-catching logo. Also, the name Star Wars provides the story with a sense of the epic—it's going to take a big ol' war to fill all the stars.
Imagine if the title had been The Adventures of Luke Skywalker—which is close to one of Lucas' actual working titles. The entirety of the story's purpose would be to tell the story of Luke Skywalker. While he's the protagonist of the film, Star Wars implies that Luke's story is one part of a much larger universe. That implication proved important later as the Star Wars story expanded with sequels, novels, and video games that focused on a cavalcade of characters, many of which never appear in the original movie.
A New Marketing Plan
During its original theatrical run, Star Wars was just Star Wars. During the 1981 re-release of the film, something peculiar happened, though, and from then on, Star Wars would be known as Star Wars Episode IV: A New Hope.
The Episode IV addition plays toward the science fiction serials of the 1930's, which served as inspiration for Lucas. By adding this, Lucas showed the world that he couldn't count sequentially. Just kidding.
He was letting fans know that the story of Star Wars wasn't one or two films but a sprawling saga broken up into various parts. The fact that it's the fourth episode means Lucas was laying the groundwork for the prequels as early as 1981. Considering the first prequel, The Phantom Menace, would be released eighteen years later, that's pretty impressive planning.
The A New Hope addition is a reference to Luke. As we learn from the prequels, the original hope was Anakin Skywalker, a.k.a. Darth Vader, but he became a tyrant/slayer of the innocent. Not exactly savior material, so the new subtitle is informing us that the task of saving the galaxy has been passed on from one generation to another, one trilogy to the other.