Out of the loop

Congress is supposed to decide yearly on how and where money gets spent in the federal government budget. If they do not pass the budget before the old one expires, the federal government cannot spend any more money, and it's operations immediately cease.

The political factions of Congress get into pretty regular spats over how the money is spent, either because not enough is going towards projects certain factions want, or too much is going towards projects that other factions are staunchly opposed to. Sometimes unrelated law bills get tacked to the side that become a point of contention too. With politics becoming more and more polarized, they reach deadlock and can't agree on the budget, and bam, you get a shutdown.

The same goes for the artificial "debt ceiling" Congress imposed upon themselves that limits how much money the government can borrow from... well, itself, basically. When they hit the cap they also cannot continue spending, so it has to be regularly increased (achieving absolutely nothing)

Either/both sides can and will try to threaten a shutdown as a bargaining chip to get what they want, but it's so overused at this point that it really doesn't help much. They still run right to the end, pass some temporary in-situ funding agreements, continue the deadlock, then go back to massive deficit spending at the end of the day anyway.