Start with the legos
A design system is a library of "lego pieces" (or technically 'components") that can be used over and over consistently to create the larger experiences. These components include everything from font styles to buttons to larger pieces like, maybe, contact cards. The smallest legos, like buttons, are called "atoms". These are put together to create the slightly larger pieces, like modals, which are called "molecules". Even larger pieces are called "organisms" - see the pattern?