Some instrumentalists, such as guitarists and pianists, are sometimes expected to be able to play a named chord, or an accompaniment (Accompaniment) based on that chord, without seeing the notes written out in common notation. In such cases, a chord symbol above the staff tells the performer what chord should be used as accompaniment to the music until the next symbol appears.
There is widespread agreement on how to name chords, but there are several different systems for writing chord symbols. Unfortunately, this can be a little confusing, particularly when different systems use the same symbol to refer to different chords. If you're not certain what chord is wanted, you can get useful clues both from the notes in the music and from the other chord symbols used. (For example, if the "minus" chord symbol is used, check to see if you can spot any chords that are clearly labelled as either minor or diminished.)