The context module lets you define conditions for when certain reactions should take place.
An example of a condition could be when viewing a certain node type and blocks should be placed as a reaction when viewing a page with this node type.
###Good resources
Context for Drupal 8 uses the built in condition plugins supplied by Drupal trough the Plugin API. So any conditional plug-ins supplied by other modules can also be used with context.
Reactions for the context module are defined trough the new Drupal 8 Plugin API.
The context module defines a plugin type named ContextReaction that you can extend when creating your own plugins.
A context reaction requires a configuration form and execute method. The execution of the plugin is also something that will have to be handled by the author of the reaction.