Renderkit is a compositional framework for the generation and processing of Drupal render arrays.
An "entity display handler" is an object that builds render arrays from entities. This may be for a specific field, an entity title link, or an entire entity displayed in a view mode with entity_view().
Some entity display handler classes take one or more other handlers as constructor arguments, e.g. to concatenate the output, or wrap it in a container element.
Besides entity display handlers, there are also classes and interfaces for lists, image derivatives, and more. All of them deal with render arrays somehow.
Many of the classes and interfaces in Renderkit will be registered as plugins and plugin types with CfrPlugin.
Additional modules can be installed to integrate some of these plugin types into existing Drupal subsystems:
EntDisP (Entity Display Plugin) integrates EntityDisplayInterface plugins as Views row plugin, Views field handler, and as a field formatter for fields that reference other entities.
ListFormat integrates ListFormatInterface plugins as Views style plugin.
CfrPreset allows to save and reuse plugin configurations, and export them with features.