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 Launch files allow you to start up and configure a number of executables containing ROS 2 nodes simultaneously.
Learn how to create a launch file that will start up nodes and their configurations all at once.
Get a more advanced overview of how launch files work.
Use substitutions to provide more flexibility when describing reusable launch files.
Use event handlers to monitor the state of processes or to define a complex set of rules that can be used to dynamically modify the launch file.
Structure launch files for large projects so they may be reused as much as possible in different situations. See usage examples of different launch tools like parameters, YAML files, remappings, namespaces, default arguments, and RViz configs.
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.