Fiscal Year is a financial accounting period of one year (that may or may not coincide with the calendar year), which is used by governments and businesses for taxation, budget planning, performance assessment, strategy formulation and other purposes.
It’s when the year ends for financial purposes. For the U.S. government, the fiscal year 2019 goes from Oct 1 2018 to Sep 30th 2019. After the end of the fiscal year, funding allocated for just that year can no longer be spent, and a law needs to be passed to spend more.
For many companies it’s just an accounting tool, but for the government if heavily effects when money can be spent.
One effect of the fiscal year is that is the U.S. government doesn’t pass a spending bill by the Sep 30th 2019, the government shuts down.