The concept of blocks has been around in Drupal since the earliest versions. Chunks of information that can be placed into the regions provided by a theme and re-used throughout your site. The system for defining blocks in code has changed quite a bit for Drupal 8 though so it's worth taking a fresh look at how to create blocks in your own module. Especially since in my experience it's one of the first things we need to know how to do as module developers.