Components are the small, individual building parts of the EC website, such as titles, text fields, dropdowns, links and buttons. When combined, these individual parts establish more complex, composable components , such as a page header, or a card.
The basic concept of component based design is that components can be used in multiple contexts. So when designing a website or webpage under ec.europa.eu, you are not designing from the page level down to the individual elements, but you design individual components and work our way up to the page level.