Some keyboards allow you to map different keys to different things depending on certain settings. So for example you could make it so that if you press ctrl+esc+enter your keyboard knows that instead of typing in English, you want your keyboard to act as though it’s a french keyboard. Some keyboards are made with this feature in mind and ship with lights that you can program to indicate which “layer” (keyboard setting) is currently active. So if you see a red light it’s English and blue it’s French. That’s probably poorly explained and only a small example of how layers can be used, but in general it’s just a preset layout that you can impose on your keyboard. The specifics of the layout are often up to you. If you’re interested there’s several companies out there making specialty keyboards and most have YouTube explanations if you google keyboard layers that are definitely clearer than my explanation.