There’s not a blog post or official documentation for the macOS app yet (I will start working on docs soon). For now you can read about it on the iOS release notes page. You can also download the Mac app from those release notes as well by pressing the “Assets” button.
The coolest part IMO is probably the “Active” sensor which determines whether your computer is currently in use by looking at a number of criteria - whether it’s locked, sleeping, if the screensaver is on, if you haven’t touched the mouse for a certain period of time (this is user configurable), etc.
And just an hour ago Zac released a new mac build with a microphone sensor. So now if you’re in a voice call or recording a podcast or something, Home Assistant can know about that and you can run automations based off that. Pretty neat.