In real-time strategy games, harassment is usually defined as using speedy units- sometimes airborne, sometimes not- to perform hit and run strikes on your opponent's forces or his base. With a bit of micromanagement, you can pull your own forces out from harm's way just before his own forces attack- and while he deals with repairing the damage done to his command (repairing buildings, replacing/healing troops, or what have you), you use the same units to strike elsewhere at his base.
As can be expected, harassment is more of a diversionary tactic than anything else. Often, a few emplacements of static defense will be able to do significant damage to your own forces without detracting from your opponent's play; the true goal of harassment is to keep your opponent concentrating on your mutalisks long enough for you to expand or to tech up to a unit capable of killing them outright.