As Shonus said. As to a bit more on the omnibus...
Under hypothetical normal circumstances (which historically have been very abnormal indeed), congress passes 12 appropriations bills covering various facets of the "discretionary" spending of the government, i.e. the spending that isn't "mandatory," i.e. the spending that doesn't happen automatically pursuant to existing statute.
Percentage-wise, Roughly 2/3 of spending is mandatory - this includes social security, medicare & medicaid, food stamps, some ongoing covid aid, etc. The other 1/3 is discretionary, of which roughly half is the military budget. So the remaining 1/6 of federal spending is all the agencies you know and love like the EPA, IRS, etc. etc. For all discretionary spending, congress has to affirmatively pass legislation every year allotting that money.
Congress somewhat arbitrarily splits this discretionary spending into 12 bills, one of which is military spending (well, 2 really) and the rest of which are various functions, e.g. "State & Foreign Operations," "Interior & Environment," or "Energy & Water." Under hypothetical normal circumstances each one would get separate floor votes and be signed into law separately, before the end of September (the end of the government fiscal year). In practice, they fail to do that by October and cobble together a bill spending agreement by the end of the year... and since it's a big must-pass bill at the end of a term, more legislation hitches a ride, including in this case some reforms to the Electoral Count Act and blocking endangered species protections for the Right Whale for 6 years, potentially leading to its imminent extinction.
Anyways, if there's a shutdown then the various discretionary agencies can no longer operate, except as absolutely needed (i.e. limited exceptions). A quirk of this is that while things like social security are mandatory, the salary and time of the staff who implement them may not be, so a shutdown will effectively disrupt mandatory programs regardless.