A guitar's action refers to the height of the strings above the frets on the fretboard. A low action means the distance between the string and the fret in minimal. (A small gap) High action is the opposite of a low action where the distance between the string and the fret is great. (A larger gap). Experienced guitar players often make adjustments or even full setups themselves. Unless you are sure you know what your doing, seek out a professional to sort it out for you. When a guitar's action is correct it is easier to play, it makes playing chords, partially Barr chords easier to grab when playing. The truss rod adjustment should be checked in the circumstance.
See string buzz.