System Idle Process is the term used for "your processor is not doing anything" it means exactly what it implies, your processor is just "idling"
98% means it is pretty much just sitting there waiting on you to do something. When you open your task manager, you will see that there are many background processing running all the time in Windows. It is perfectly normal, most of them must be running to support your OS. Among these things are programs like your antivirus, automatic updates, system restore is actively tracking everything you do and creating restore points, scheduled system maintenance programs, many things can cause your memory and processor usage to go up and down even when you "think" the PC is doing nothing. In reality, it is rare that your PC is ever doing absolutely nothing.