Appropriations are decisions made by Congress about how to allocate federal spending. This process occurs every fiscal year, and the details can be hard to understand. Here's a (very simplified) synopsis of how appropriations is supposed to work, though there are often political hiccups:
Step 1: After working with the agencies on their needs and considering the administration’s priorities, the White House submits the president’s budget to Congress early in the calendar year to coincide with the State of the Union Address. This triggers the start of the appropriations process for the upcoming fiscal year.
In addition to the topline president’s budget, each federal agency releases a Congressional Justification report that provides further detail as to how the proposed funds would be used to address current needs.
Congress uses the president’s budget and the associated justification reports as a template (or not, depending on Congressional priorities) to draft appropriations, with jurisdiction over federal government resources divided into 12 bills.
Step 2: House and Senate appropriations committees collect requests from their colleagues in Congress in the spring and, based on those requests and their own priorities, write, amend, and mark up the bills in late spring/early summer. The House writes their bills first, followed by the Senate, and then the two chambers agree on a compromise “consolidated” version which is then agreed to by both chambers.
Step 3: Congress passes the consolidated 12 appropriations bills and the President signs them into law, funding the government before the previous year’s funding expires at midnight on September 30.