ROS (Robot Operating System) is a general purpose robotics library that can be used with PX4 for offboard control. It uses the MAVROS node to communicate with PX4 running on hardware or using the Gazebo Simulator.
This section contains topics about using ROS for offboard control with PX4.
ROS is only officially supported on Linux platforms.
The easiest way to setup PX4 simulation with ROS (on Ubuntu Linux) is to use the standard installation script that can be found at Development Environment on Linux > Gazebo with ROS. The script installs everything you need: PX4, ROS, the Gazebo simulator, and MAVROS.
If you just need to install ROS then follow the ROS Melodic installation instructions for your platform.