You're reading the documentation for an older, but still supported, version of ROS 2. For information on the latest version, please have a look at Iron.
ROS 2 is a middleware based on a strongly-typed, anonymous publish/subscribe mechanism that allows for message passing between different processes.
At the heart of any ROS 2 system is the ROS graph. The ROS graph refers to the network of nodes in a ROS system and the connections between them by which they communicate.
These are the concepts that will help you get started understanding the basics of ROS 2.