It’s the ability to peak into a VR experience from your mobile phone, you can track someone in an HTC Vive, this makes use of ARkit

Read the full blog post at Owlchemy Labs here https://owlchemylabs.com/owlchemy-mobile-spectator-ar-spectator-camera/

Owlchemy Lab Report: Mobile Spectator - AR Spectator Camera Experiment
As the saying goes- science never sleeps! Today, we're excited to share an R&D project fresh from the Labs. After…owlchemylabs.com

At Oculus Connect 5 John Carmack mentioned that users of the Oculus Go will be able to see what Go users are seeing from their phone.

It was also revealed at Connect 5 that Oculus Insight runs on an iPad and allows people to see into the VR experience of Oculus Quest users.

The general trend here appears to be that phones and tablets in general will be able to see into VR experiences, improving the experience for those outside the VR headset who would like to have a better idea of what is going on inside the headset.