Design components are specific examples of design elements that might exist within a given context. They can be thought of like building blocks for design, which are often documented within a component library as part of a larger design system. Examples of design components could include elements such as buttons, headings, icons, form fields, and menus.
To dive deeper into one example, a button designed in accordance with the style guide for a specific website or brand would qualify as a component. Buttons are clickable elements often used to highlight certain calls-to-action or links on a website, and are a common design pattern used across all kinds of websites and applications.