"Holotars" are recordings of a player's avatar and microphone of up to 15 seconds. The recordings are created and played back with a Holotar projector from the Sandbox Machine in Custom Rooms (in Sandbox mode). To use a Holotar projector, get one from the Sandbox Machine, place it somewhere in a custom room, then laser point the button to start recording your movements and voice.
Holotars are saved with a custom room, so any visitors will see them when they get close to the Holotar projector. For finer control of the playback, the Holotar projector can be connected to Circuits. Use the Settings tool of the Maker Pen to access further (playback) options and to delete the recording.