Or "Allahu Akbar", (الله أَكْبَر) means (roughly) "God is the greatest" in Arabic. The phrase is called the Takbir (تَكْبِير).
If the person shouting this is heavily armed, you might want to get out of the way: or they could be pleased to see you, upset with the cricket scores, or have just received good/bad news. Confusingly, it's what you'd say if you were a perfectly innocent civilian who's just had a fright, and it's also what you'd say if you were about to kill a few of the enemy in the next couple of seconds
When it's being chanted as part of a celebration/demonstration there will often be one person who calls out, "Takbir!" to which everyone replies, "Allahu Akbar!" as a sort of religious hip-hip-hooray.
If you say it 22 times while looking at an Iranian flag, you've read the flag!