A processor core is a processing unit which reads in instructions to perform specific actions. Instructions are chained together so that, when run in real time, they make up your computer experience. Literally everything you do on your computer has to be processed by your processor. Whenever you open a folder, that requires your processor. When you type into a word document, that also requires your processor. Things like drawing the desktop environment, the windows, and game graphics are the job of your graphics card — which contains hundreds of processors to quickly work on data simultaneously — but to some extent they still require your processor as well.