You're reading the documentation for a development version.
For the latest released version, please have a look at Iron.
Conceptual overviews provide relatively high-level, general background information about key aspects of ROS 2.
See the ROS 2 citations for more explanation of concepts and citable resources.
For a brief video introduction to ROS 2, see this community contributed content:
Getting started with ROS Part 1: Nodes, Parameters and Topics
Getting started with ROS Part 2: Services and Actions