I’ve read two articles on thread and still don’t know what it is. Something to do with smart homes?

its a low level protocol like WiFi on top of which Smart Home protocols, like Matter, run. It allows for IoT devices to be managed, registered and configured completely local with a hub (any iPad, HomePod and so on) and requires no servers. It is private by design and more secure in some ways, as no one but your hub can control the device. Currently a lot of IoT devices rely on a server that registers and controls them, and is in the hands of a random company you need to trust.

TLDR: privacy, security.