Action is the distance (or height" of the strings compared to the fretboard. Lower action is easier to play on, but you can get string buzzing if it is too low. Higher action is harder to play on, but can sometimes give a little more sustain to your notes.

There is usually a "standard" height, but it goes by personal preference as to what someone prefers. I like my action to be as low as possible without causing string buzz so that I can play faster.