If the WordPress core is the framing and foundation of a house, the theme is the exterior finishes and add-ons.
In it’s most basic form, a theme is a folder with some files in it. That includes style.css, functions.php, images, and page templates among other things.
The main thing to understand here is that you have a folder with some files in it, and to customize the theme you don’t ever touch those files. Instead, we create a separate folder with our own files we make changes to, called a Child Theme.