Layers are one of most powerful features an image editing application can have. They are important to any editing workflow as they can be stacked, moved, masked, subjected to different Blend Modes and Opacities to meet your artistic goals and all without modifying your original image.
When we find what we consider to be a comprehensive and clear introduction to a topic, we see no point in duplicating effort, so the following is a link to the University of Michigan Library’s Introduction to Layers. Although the document refers to Adobe Photoshop CS6’s implementation, the basic concepts and conventions apply to most applications which have layers implemented (the image to the right shows a Photopea Layer stack)
After using the University’s page to get the basics, photoshopessentials.com have an excellent in-depth set of pages which obviously go into far greater detail.
If you have any actions, presets, tips, worked examples you would be happy to contribute and join others helping people get off to a good start with their photo processing, pay your knowledge forward !