The sensors screen lists most of the sensors that are available to the FMU (internal or external). It allows you to start the calibration process for the listed sensors. The QGroundControl user guide has all information you need about calibrating the sensors. This step is very important for stable flights, it is required to do the calibration at least once and it should be redone whenever the drone starts flying less stable!
You can check whether the sensors are properly calibrated using the Analyze widget, which can be found under Widgets at the top of the screen. This is a tool in QGroundControl that allows you to plot sensor values of sensors inside of the RDDRONE-FMUK66 in real-time.
You can check the calibration of the sensors inside of the FMU by putting the drone on a flat, horizontal surface, and looking at the ATTITUDE.roll and ATTITUDE.pitch variables. If they are larger than 0.02 (both positive and negative), you should consider recalibrating the Accelerometer, Gyroscope and Level Horizon.