Arabic for "Allah is Most Great." Used on many occasions, be it the call to prayer, when slaying animals for food, etc. as a form of confession or a calling out the name and rule of God over everything.
Very prominently it is the the rallying cry or battle cry among Muslims both in past and present times. As such it is a statement asserting at the same time the superiority of Allah, Islam and Muslims over everyone else. See for example how it is frequently reported in the wars of the Muslims against the infidels in The Caliphate by Sir William Muir.