A Guest Light is a LIFX light available on your Wi-Fi network, but owned and configured to another LIFX Cloud account. This is generally used by family members, friends or a guest of the owner of the LIFX Cloud account that the lights were initially connected to.
When your device is connected to a Wi-Fi network with LIFX lights connected, your LIFX app will be able to access them, turn them on and off as well as conduct other basic tasks such as change colors and brightness.
However, you will only have access to these features. Renaming lights, editing them, and creating Schedules or other LIFX Cloud connections, will not be possible.
If you have set up these lights, but they appear as guest lights, please ensure you have logged into the LIFX app with the correct LIFX app email/password. If you cannot access an account that the lights were previously connected to, you can reset the light, and reconnect it to your network, using the current LIFX app account you are using.