What is U.M.A. ?
Unity Multipurpose Avatar aka U.M.A.
Very simply put it's for character creation / customization.
more technical explanation
It allows for an optimizes multiple layers of clothing instead of the current one layer that we have right now. Think of it like being able to put on a shirt, coat, and scarf at the same time whereas right now our characters basically take off their shirt to put on a coat. There's a lot more than can be done with it, that's just one example. It's also been around for years and well supported so there's tons of different character models, etc. available for it along with countless accessories like clothing, etc.
https://assetstore.unity.com/p…3d/characters/uma-2-35611