A large majority of what Congress does involves introducing, debating and passing authorizing legislation. Authorizing legislation provides for the authority to do something. It may create a new Department or Bureau, devise a new program, and outline responsibilities of some government entity. Basically, authorizing legislation is what most people think of when they think of legislation. Most of the regular committees of Congress only deal with this type of legislation. If you want to get the Federal Communications Commission (FCC) to change how your cable provider bills you, you ask your Congress person to introduce a bill to do so, or have him or her amend some other bill to get it done. Everything in government has been authorized in some form at some time. The FCC was initially authorized way back in the 1930s and people still go back to that dusty bill to check its authority. Social Security, Medicare, Medicaid, every Cabinet Department, the Army, Navy, the Post Office - everything was created by authorizing legislation in Congress at some time. Maybe it was a hundred years ago, but it was authorized. In fact, not only authorized but re-authorized over and over again.
The other side of the coin is appropriations. Appropriations bills do not authorize, (though in some cases authorization bills can appropriate, though for simplicity's sake, let's keep the two separate). In order to appropriate money, the program (where the money is being spent) must be authorized.