HomeKit is Apple's framework for home automation
HomeKit is a security / review / integration framework that integrates Siri and iCloud / AppleID authentication and storage for accessories.
Some notable uses are "smart home" devices like locks and door controllers, lights, security cameras, internet routers and firewalls, heating and cooling, audio visual and more can be integrated with Apple hardware and services.
Questions on code level questions are likely off topic here so this tag shouldn't be used for developers unless the usual developer tags are also applied. Questions on setup, security, requirements, troubleshooting are all on topic here. As long as the other requirements for off site recommendations or shopping questions are honored, questions that meet quality standards for the site and relate to HomeKit can use this tag as well.