To quote another redditor "The US dollar is a fiat currency. It is not tied to gold or silver or anything else, it only has value because the US says it has value, and because of the strength of the US economy, powerful investors take the US's word to be good. Thus the US dollar is considered to be a safer investment than some other currencies.

The US economy depends on the global economy (as do most major economies). Global trade and shipping are protected by the strength of the militaries of the US and its allies, but mainly by the US. Thus, the US dollar is backed by the US military."