The Mirror app is a simple app designed to send commands to one or more "replica" (secondary) devices based on events from a "primary" device.
NOTE: The "mirroring" is one-way, from the primary device to the replica device(s). Only changes registered on the primary device will cause commands to be sent to the replica devices. Changing the replica devices will not cause any changes elsewhere (nor will running commands on the primary device that do not actually cause state changes or that do not correspond to one of the events listed as "forwarded" in the interface after all the devices are selected).
Select Create New Device Mirror... to create an instance of Mirror (these child apps will be called "Mirror Me") to make one or more switches, bulbs, dimmers, etc. follow another. This type of mirror is intended to send switch, level, and color/color temperature commands based on events from one "primary" device to one or more "replica" device(s). It was specifically designed for use with controllers that have color wheels or slider-type surfaces, as some remote and hardwired control devices do, though it can be used for any case where you want one or more devices to "follow" the reported state of another.
Select Create New Lock Mirror... to create an instance of Mirror (these child apps will be called "Lock Mirror") to make one or more locks follow one or more other locks. This type of mirror is intended to send "Lock" or "Unlock" commands to the replica lock(s) based on events generated from the source lock(s) — e.g., when a source lock is unlocked and reports that event to the hub, an "Unlock" command will be sent to the replica locks, and similar with the "Lock" command.
lockattribute you can see under "Current States" on the device detail page).