A preset is a saved LED light configuration that you can recall later without having to set every details again. For example, you can set segments for a certain desired effect, define a color or an effect for each segment. You can then save these settings in a preset and call them up again and again with only one click when you need them. You can also set in WLED settings that a certain preset is automatically called up when the controller is started. Or you can connect a physical button to the controller and then assign a preset to that button. Or you can set in WLED settings that a preset is activated at a certain time of day.