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 Humble.
ROS 2 Launch files allow you to start up and configure a number of executables containing ROS 2 nodes simultaneously.
This tutorial will show you how to create a launch file that will start up nodes and their configurations all at once.
A more advanced overview of how launch files work and of the resources and documentation available for them.
Substitutions can be used to provide more flexibility when describing reusable launch files.
Event handlers can be useful for monitoring the state of processes or to define a complex set of rules that can be used to dynamically modify the launch file.
This tutorial describes some tips for writing launch files for large projects. The focus is on how to structure launch files so they may be reused as much as possible in different situations. Additionally, it covers usage examples of different ROS 2 launch tools, like parameters, YAML files, remappings, namespaces, default arguments, and RViz configs.
You can also use XML and YAML to create launch files. You can see a comparison of these different ROS 2 launch formats in Using Python, XML, and YAML for ROS 2 Launch Files.
If you are coming from ROS 1, you can use the ROS Launch Migration guide to help you migrate your launch files to ROS 2.