This is the theme of:

The Force

Description: Whenever the Force is being used or is exerting its influence, this theme can be heard. This is one of the few themes that is played repeatedly throughout all six films. Many different instruments play this theme. In this example, it's played by French Horns followed by the string section. This particular clip is a scene in which Luke is frustrated by his uncle keeping him from pursuing his dreams. He watches the two suns of Tatooine set, while he ponders his destiny.

This theme was first introduced in Star Wars, Episode IV: A New Hope