It is a connectivity language that keeps these smart things in communication without Bluetooth or wifi.
Still haven't found anyone that can tell me what Thread is and how it is any better or different from what we've had all along.
The communications standards that HomeKit and other home automation work on are designed to be one-on-one broadcasting standards, but not really designed to cooperate. It's this lack of cooperation and rebroadcasting that causes most of the failures in home automation, and Thread is designed to both reduce failure but also extend the distance and complexity of your network, without having to do anything but add Thread capable devices.
The TLDR is that both WiFi and Bluetooth were designed for other things, and are being used for home networking, and that has drawbacks. Thread was designed to be use don home automation-type problems, and so doesn't have those drawbacks.
WiFi and Bluetooth both transmit instructions from HomeKit router to device, and the devices use either Bluetooth or WiFi to transmit back.
As a peer-to-peer broadcast system, both have drawbacks. WiFi gets slower/less effective the more devices are on the network. WiFi communications from the devices requires a relatively high amount of power too. Bluetooth is much more power efficient, but the range isn't designed to be as far as WiFi. (This is why headphones start to drop audio when you get a few dozen feet from your phone.)
Thread is a communications standard that uses repeaters to re-broadcast the signal. It's essentially Mesh-Wifi and Bluetooth had a baby.
Each router on the network—and a router can be a outlet device like this, or a bulb, or a HomePod mini, etc.—makes sure the signal is re-broadcast until it has arrived at its destination device, and can confirm that that has happened.
This means that the more Thread devices that act as routers that you add, the better your communication between all of your devices get. And since Thread is a cheap standard to implement, you end up with things like $20 lightbulbs that have Thread routers built into them.
With thread there's lower lag times between event triggers and the action, fewer "I'm sorry I haven't heard back from your devices", and no need to use WiFi in order to connect devices, which means your home network performs better too.