Re: Surely this is a joke
Most of the U.S. government is funded on a year-to-year basis, with the fiscal year ending 30 September. Congress is supposed to pass funding bills for the next fiscal year before the end of the current fiscal year. If that doesn't happen, by law, the parts of the government that did not get funded are shut down until new appropriations bills are passed. (Someone, somewhere, thought this was a good idea.)
There are exceptions for functions that are essential to the protection of life and property (the definition of which seems to be left as an exercise for the reader,) so the Weather Service, air traffic controllers, half of the Defense Department, etc are still on the job. Also, things that are funded through user fees or other mechanisms that don't expire at the end of the year are still open. This includes things like the mail, courts, passport processing, Amtrak, the Patent Office, some benefits programs, and the like.
There were a bunch of shutdowns in the 1970s and 1980s as well as a couple in the mid-90s. So this is not without precedent, but it hasn't happened anytime in recent memory.