It's transferable debt.
If I give Paul a loaf of bread and in exchange he writes "Paul owes you one" on a piece of paper, that's money. I can trade that with someone else for some good or service and we'll have to haggle over how much "one" favor from Paul is actually worth.
The dollar is just "the US government owes you one". How much is "one" worth? That's for the market to decide, and largely depends on how much real world goods and services there actually are along with how many "ones" the US government has issued. Although, before 1971, it was explicitly defined as "for every 35 'ones' you have, the US government owes you 1 oz of gold". Now, the value of "one" floats in the market and is constantly devalued by design.
In recent days, the world's markets have decided that "the Russian government owes you one" isn't worth nearly as much as it did last week.
Commodity money like gold or bitcoin don't rely on trust in a government's ability to pay it's debt, but rather trust that a sufficient number of people will always desire the commodity for whatever reason. That's still debt, but more a more indirect, coercion free form of debt.
For gold, that reason is because it has a long history of established scarcity and has some utility as jewelry to signal wealth to others.
For bitcoin, that reason is because it has theoretic perfect scarcity, can be instantly transferred to anyone on the planet with a phone, can be stored in your brain by memorizing the dozen words of your seed phrase, and many other fascinating programmable properties that have yet to be fully explored.