The Render API consists of two parts: structured arrays that provide data and hints about how that data should be rendered, and a rendering pipeline that can be used to render these arrays into various output formats. It is used for all content output for display by Drupal, as well as for describing and controlling forms that collect user input, and directing how that input is handled.
The Render API serves as the glue between modules which store and manipulate data on the backend, and themes that display that data to users. As such, it's important for both theme and module developers to understand at least the basics of how the Render API works, the difference between elements and properties in a render array, and the concept of callback functions.
We don't anticipate any major changes to how the Render API works in the Drupal 8 cycle. Writing a render array hasn't changed much between Drupal 7 and 8. The concepts like elements and properties are still the same, with the major difference being that render elements are now implemented as plugins. Resources labeled for Drupal 8 should apply to Drupal 9.