the ability to play a piece of music that you've never played before simply by reading it off of a page of written music. In many ways, this skill is no different than an actor's ability to convincingly perform a section of dialogue by reading it straight off the page.
Sight-reading is an important and necessary skill for music majors. Quite often, they will be asked to learn pieces of music in a very short amount of time. The ability to sight-read is critical to the ability to function within a performance ensemble. Truly understanding how music works in all its forms gives you unlimited potential to continue pushing boundaries and developing musically.