Arabic for "Allah is great" ie greater than the god of the infidels being attacked. it has been used by Muslims since the beginning of Islam when Mohammed was commanding his followers to kill, rape and plunder his enemies.
A common used stereotype against Syrian people or people that wear hijabs, also used in Dank Memes, usually said or uses in the time of a bombing, explosion, or somethingexploding.
Literally means, God is Great in arabic, and is used to praise God.
Some people think it's a joke and mock us by making bomb jokes with "Allahu akbar" in them...