Referring to distinctive UI elements that are used over and over throughout the UI. A few examples include buttons, form inputs, avatars and tooltips. Another way of looking at it: A component is a UI element that you could 'drag & drop' into an artboard to make up the UI. Consistent UI components make a huge difference in guiding a user through a product, smoothly and efficiently.
Everything documented here is an extension of, and in addition to the 'Digital Foundations ↗' documentation that covers all our digital products.