I know that the phrase "Feuer Frei!" means "open fire", or literally "Fire freely".
I think the germans use it to essentially say "fire at will".
I've heard the word "Feuer" by itself without the Frei as well, although it is the flame type of fire. It's used the same as in english, where fire means both to shoot and a burning flame.
Chances are the german is yelling "Feuer!".
to pronounce it, the "Feu-" part is a "Foy" sound, like the town in band of brothers. Like Toy with an F. the "-er" at the end makes an "ah" sound. So its like "Foy-ah".
I'm sure that this applies to rifled projectiles like guns, tanks, and arty. However, I'm not 100% sure it is used for other types like mortars, torpedos, bombs etc.