In the game of StarCraft, a Missile Turret is the Terrans' static anti-air defense (the Bunker can also be used for defense and was in fact intended for this, but they aren't used as often). Missile Turrets are used for detection and since they only cost 75 Minerals, they are often used as semi-mobile detectors (because their low cost allows you to build them as you move your army along without acquiring higher technology, like Comsat Stations or Science Vessels when a Terran is Tank Pushing (for example, one would build Turrets as their army advances toward a Protoss base to detect Dark Templar or stop Zealot Drops from destroying their Siege Tank force). That said, I have no idea why someone would do a Turret rush. Only Zerg have early air units (the Overlord) and if a Terran player is able to build Turrets to destroy Overlords, their opponent obviously isn't very skilled and could be killed by anything.