The short elevator pitch in my "unofficial" over-simplified own words - "ROS is framework to program complex robotics applications that encourages modularity and shareability of modules. ROS is to robotics development what Django/Rails is to web development."

ROS2 is version 2 of the original ROS framework.

As a results of the ROS framework, a number of modules (packages in ROS-talk) for complex robotics algorithms, functionality has been implemented by companies / the community and shared among each other.

For instance, Hadabot reuses the Navigation package (along with many others) that has been pre-impremented which allows me to employ many robotics functionalities without having to re-invent the wheel.