It's a tongue in cheek reference to the Federal Reserve "running the printing presses", generating as much fiat as necessary to support the US and global economy. With the US dollar not on the gold standard [1], or tied to something physical resource, the Federal Reserve can create money out of thin air without constraint. Ergo, the "money printer goes brrr".
https://www.nytimes.com/2020/03/23/upshot/coronavirus-fed-ex... (The Fed’s Message: The Money-Printing Presses Are Fired Up and Ready to Go)
[1] https://en.wikipedia.org/wiki/Gold_standard