This is the online resource for out-of-the-box and community-supported apply-device-config scripts.
They are custom scripts, which can be installed into the /usr/lib/mender-configure/apply-device-config.d directory. They are then executed by the Configure Update Module, with the expected configuration parameters from the Mender server. In this way, you can extend Mender-Configure to you needs and keep your device configuration in sync.
You can read more about how the Configure add-on work in our official documentation.