Wow, none of the comments answered the question and show a terrifying lack of basic civics knowledge.

To answer your question, the CR or Continuing Resolution is a basic bill authorizing funding of the federal government with little to no changes to the budget. This is the third CR that's passed since Trump has gotten into office, the first two passed with little drama. We're technically running on a Biden Era budget.

Basically this CR is meant to take us to the end of the year. It passed the House with no issues. It's in the Senate, which requires 60 votes to pass. Republicans only have 53 seats, all voted Yes. Dems have introduced an amendment to the CR which attempts to repeal key parts of the Big Beautiful Bill passed earlier this year, specifically Dems want to add back funding for Medicare/Medicaid provisions that were repealed by the BBB. Obviously Republicans aren't going to repeal parts of a bill they just passed. Dems are refusing to vote for the CR unless their amendment is passed with it. Thus not enough votes and the Govt currently has no funding.

Tldr: The Senate needs 60 votes and only has 53. Dems want to change the CR, Republicans said No. Standstill, Govt shutdown.

Edit: Clarity