So to give an explanation thing. Libra deals with scales, and since that is Terezi’s sign, obviously this Scale is hers.

Scales are used for Balance, Fate, Justice, etc. And seeing how one side is a glowing blue egg and the other is a demon skull thing over a black box, I can assume it is the Balance of Good and Evil.

Black Box is a term used for all the secret information on what has happened during an event/dark undertelling of what truly happened. Originally used in airplanes to record all information during a flight (audio, fuel line, etc), and is used to determine the cause of a crash. Current use of Black Box is all the dirty little secrets that have been collected to make a thing a thing. Think of it as a modern Pandora’s Box.

So if Terezi ever takes her Lusus to hatch/live with, it would tip the scales to death/evil, releasing all of the information/secret shit that SBURB has been hiding for who knows how long.