ROS runs on top of Linux and structures software into nodes, small programs that each do one job such as reading a lidar, building a map or driving the wheels. Nodes exchange data by publishing and subscribing to named topics, so a camera driver can be swapped without touching the rest of the system.
Around that core sits a large ecosystem: drivers for common sensors and arms, navigation and manipulation stacks, the RViz visualiser and the Gazebo simulator.
ROS 2 is the current generation, rebuilt on the DDS communication standard with better support for real-time behaviour, multiple robots and security, which is what made it usable in commercial products rather than only on lab prototypes.
