Well it is in Dutch so translatable.
Top knob is heater voltage. either off, 2V, 6V3 or 8V. Probably AC from a set of windings on the main transformer. Used to check tubes and rejuvenate low emission heaters.
Lower knob.
Leak test, emission test, shorted electrode test then a low and high EHT test. Basically tests the tube that it will act as a basic rectifier and the grids can control the emission of the tube.
The 3 neon's show heater leakage, G1 current and G2 current by lighting on current flow due to shorted electrode assemblies. Basically you connect the tube using the common base pinouts for CRT's and then connect the EHT to the anode connection on the side of the tube then power the device and select the test then press the button. Read the values off the meter and compare to manual for readings.