He means active disk usage is 100%. It's loading programs and stuff.
First, open the start menu and search for and run "disk cleanup" and check every box to clean up some junk.
Then, open "programs and features" from the control panel, and remove any programs you're not using. Try to be as aggressive as you can here. Even if you think you might use a program some day down the road, go ahead and remove it to free up some of the space. More used space = more time to start the same programs that would take a few seconds on a nearly empty disk.
Another thing you can do is either run the built in defragment program, or I recommend iobit Smart Defrag, which has options to prioritize files so the most used files are on the edge of the disk where the surface speed is faster, so files can be accessed a bit quicker. Do not do this if you have a solid state drive. iobit and I think windows (maybe) will warn you not to do it if it's an SSD