Themes are much more than just layout with their respective HTML, CSS, and JavaScript. Without content, we would not have much to display. In Drupal 8, a lot of content has been moved into blocks that are then assigned to various regions.
Think of blocks as small sections of content. These blocks can contain a menu, a search form, a listing of content, plain HTML, and more. Drupal 8 uses this content within the Block layout system that makes up a good part of a website.
If you are familiar with blocks in Drupal 7, you will be happily surprised to find that blocks have matured and are now fieldable, similar to content types. This new implementation of blocks also allows the same block to be reused and displayed using different view modes. As we explore the Block layout, we will learn how to place blocks, configure them, and create custom blocks.