Yeah, the 2 sensors. Luckily, these come with the bundle
The sensors are basically web cams. The Rift headset and controllers all emit short, infrared pulses, which are invisible to us (although you may be able to see using your phone's camera). The sensors use these lights to track position. It's known as constellation tracking.
The 2 supplied are intended to be placed in front of you, on either side of your monitor. But that has the problem where if you turn around, the sensors won't be able to see the controllers. To help with that you can try placing one infront and the other behind or in diagonal corners, usually higher up and pointing a little downwards. This still leaves some blind spots where your arms may block line of sight to a controller. If that is an issue then you can purchase a 3rd sensor. I believe the system supports up to 6 sensors, but most of us use 2 or 3.
https://www.roadtovr.com/new-oculus-update-embraces-360-room-scale-adds-mixed-reality-capture/