Copresence appears to be aimed at allowing Android, iOS, and Chrome devices communicate with one another and allow for exchanging files, photos, directions, messages, or other content.
Devices would use location data or Bluetooth to establish a connection, then transfer files through Wi-Fi or Wi-Fi Direct. It appears as though Google might have plans for the product beyond simple file transfers; meaning it could be deeper syncing of data between devices, similar to the new Continuity features between iOS and Mac after the release of OS X Yosemite; or it could be more seamless casting to Android TV or Chromecast; or it could be something else completely different.
At this point we have No clue, but let’s wait and see. I’m excited. Are you?