Components are blocks that you design once and are used in a variety of different places across your store with the same content, such as banners.
You can't add new components in Instant Commerce.
Components versus pages
The difference between components and pages is that the components you design are always shown with the same content. E.g., your email pop-up will always display the same content, no matter where it is shown in your store.
Pages, on the other hand, are used as templates to display different variations of the same basic page. All your products are displayed on their individual pages, but you only create one Product Detail page, for instance.