system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why might the government be shutting down (in the US)?
gpt
There are two major parties in the US: Democratic and Republican. 

The Republican Party has decided that the entire purpose of their existence at the Federal level is to undo a Healthcare Law called the "Affordable Care Act"  AKA OBAMACARE which was passed early in Barack Obama's presidency. 

There are two relevant branches of Federal Government in the US: The Senate and The House. The House has a Republican majority, but the Senate has a Democratic Majority. This means that the government is incapable of getting anything done. The House has voted 40 times to repeal Obamacare, and the Senate has of course prevented this from passing, 40 times. 

The implementation of the budget for this year will bring into effect the funding for the final stages of Obamacare. The Republicans see this as their last chance to repeal the law before it takes effect across the country and becomes much more difficult to do away with. They realize that their chances of getting the law repealed are exactly the same as the last 40 times they've tried, so with their house majority they have passed a law which basically states that they will not allow the budget to take effect unless the Senate agrees to Repeal Obamacare. This would have the effect of halting funding for most federal government services; hence the "government shutdown" that is being talked about. 

tl;dr: the republicans are holding the budget hostage in an attempt to get Democrats to agree to defund Obamacare. If Democrats call their bluff (which they are doing), the government will stop paying its bills.