Congress is charged by the Constitution with making decisions about how to spend public money. In practice, these spending decisions are split into two parts: authorization and appropriations. This document briefly explains the difference between these two steps, using the 2018 budget as an example.
“Authorization” is done by Congress via legislation that “can establish, continue, or modify an agency, program, or activity for a fixed or indefinite period of time,” per the Congressional Research Service. In other words, authorization is Congress saying that money can be spent on a given item — not that it necessarily will be spent on that item.
When Congress passed its two-year budget deal in February 2018, it authorized $1.3 trillion in government spending. But that just set new levels for what the government was allowed to spend; in order to actually spend that money, they had to pass another bill in March 2018 that appropriated funding based on the spending levels they had set in the authorization bill.
“Appropriations” are done by Congress via legislation that authorizes agencies to make payments from the federal Treasury (i.e. it allows them to spend the money that had previously been authorized). Appropriations bills are ordinarily passed each year, but in recent years it has been common for Congress to fund the government “on autopilot” via continuing resolutions that simply allow agencies to continue spending the same amount of money they were spending under the previous funding bill.
When Congress passed the budget deal in February 2018, they included a continuing resolution that funded government agencies at the same level as before for six weeks so they could have time to work out an appropriations bill. Then in March, they passed a $1.3 trillion appropriations bill that set new spending levels for government agencies through the end of September 2018. These spending levels aligned with what was authorized in the budget deal from February 2018.