Chat Heads is a feature that allows you to access an SMS or Facebook Messenger conversation from any screen on your Android device by tapping on small circles that contain a profile pic of whoever it is you're conversing with.
There aren't too many Android devices that will be getting access to Facebook Home today once it's released in the Play Store, but by installing the standalone Facebook Messenger app (or updating to the latest version if you already have it installed), you'll be able to use Chat Heads.
Once you've downloaded the latest version of Facebook Messenger, you'll need to make sure the Chat Heads feature is enabled. You can do this by launching the Messenger app, tapping on the menu icon, tapping "Settings," and then selecting "Notifications." At the bottom of the list you'll see a check box to enable Chat Heads. Or if you'd rather not have your friends' faces intruding on your screen at any given time, you can uncheck the box to disable Chat Heads.
If the feature doesn't start working right away, you may need to restart your Android device. On my Nexus 4, I didn't see Chat Heads show up until after I restarted.
To bring text messages into the Chat Heads feature, you'll need to enable Facebook Messenger as your text messaging app. To do this, launch the Messenger app, visit the settings screen, and select SMS/MMS from the list of options. Check both boxes on the SMS/MMS screen and your text messages will start funneling into the Messenger app.
It's a good idea to go back into the default Messaging app and disable notifications to prevent duplicate notifications when you receive text messages. You can do that by launching the Messaging app, visiting the settings page, and unchecking the box next to "Notifications."
You'll also want to make sure to set photos for the contacts who currently don't have a picture in your address book. This will ensure that their Chat Heads bubble looks great and you're quickly able to identify who it is you're talking to.
Download the latest version of Facebook Messenger from the Play store to start using Chat Heads right now.