米 means "rice," but remember how it can mean "America" too? Well, all America is exactly what it sounds like: all America. Just keep in mind that we're not talking about America as a continent here. This word specifically means the entire USA — no more, no less.

This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.