Allahu Akbar is a Muslim saying. The exact words in Arabic mean "God is greater", but people know that saying "God is great" in the right time and place means the same as "God is Greater." "Allahu Akbar" is the first part of the Adhan, which people hear 5 times a day where Muslim people live together. Saying "God is Great" tells people in a few words what Islam believes and teaches.
This Arabic phrase was added to the Iraqi flag, in Saddam Hussein's handwriting, in 2001. Muslims say it before changing positions during their prayers. Allahu Akbar was the anthem of Libya during the rule of Muammar Gaddafi.