Technically speaking, the "great arsenal of democracy" which FDR refers to in his speech is U.S. industry—specifically American industry becoming the chief supplier of munitions to help the Allies defeat the Axis.

But remember, kids—everything's a metaphor.

So the "great arsenal of democracy" also referred to America's obligation to fight for those whose freedom was under attack. The U.S. had access to an incredible amount of resources and was absolutely able to produce the kinds of weapons and ammo that could win the war.

But, Americans also understood the value of freedom, and were willing to sacrifice their lives to help others get it, and also to help them keep it. Because that's just what America does.