This package fills the gap between getting raw images from a camera driver and higher-level vision processing.
Documentation is hosted in the ROS 2 API docs. The image_pipeline documentation includes an overview, details on camera_info, and links to the documentation for each individual package.
Not every aspect has been ported to the new ROS 2 API documentation yet, so there is still additional (partially outdated) information in the ROS wiki entry.
If you are using an Nvidia Jetson platform, consider using modules from Isaac Image Proc - a collection of hardware accelerated image_proc features for the Jetsons.