It's a raw ADC differential reading of the output. It translates directly into the large number below it (X.Y n/u/mA).
It's more a remnant of development, but it can be helpful with determining ADC offsets when the n/u/A reading is actually 0.0.
You can always look at the code and see exactly where it comes from and how it translates into the reading, or you can remove/edit/change it to something else if you'd like.
If you reflash the unit - mind the recommended calibration values on the little label you receive with the unit. Upon reflashing it - if the self calibration is much different from those values, you should probably hard code the recommended calibration values using the directives at the top of the sketch.
PS. Be careful with those whiskers between the VCC and GND of the OLED!