There are lots of Federal agencies under the Executive Branch, The heads of those agencies are often political appointees, who are appointed directly by a President and are not expected to remain in their roles if the party in power changes. However, the actual work is done by career civil servants, who are hired based on merit and not their political connections.
Conservatives see this as a problem, because these career civil servants may have been hired by prior administrations, and they see every appointment by the opposition as tainted. They want to wreck the merit-based system that these people are hired under, subject all career government workers to loyalty tests, and fire the ones who don’t meet those tests.
This is, of course, a recipe to decimate the functioning of these agencies and make them totally ineffective. But that’s the whole point. This “deep state” they keep railing about are simply people who put their commitment to the country over their commitment to any one party or one President. So they must go. And it will shrink all these agencies in the process, which they see as a win.
This is an analysis from a Liberal advocacy group but I think it is an accurate description of what these chuckleheads are really up to: