The lamp is a high impedance tungsten filament thing that runs maybe just red hot with a very few volts.
The idea is that as the oscillation amplitude builds up, the temperature of the filament increases. The resistance of the tungsten filament increases fairly dramatically with temperature.
The lamp is placed in series with a resistor which forms part of a voltage divider in the feedback path.
As oscillation amplitude increases, the lamp glows brighter and the change in resistance stabilises the amplitude.
The clever thing about it is the thermal inertia of the filament is long enough to not change during one cycle of oscillation, so it does not distort the waveform, but its fast enough to rapidly stabilise the amplitude.
It works down to a few Hz where the lamp may be seen to visibly flash, and distortion increases. But at higher frequencies it works pretty well. Back in the vacuum tube era, before silicon diodes and Jfets, it was pretty common.
*edit*
You beat me to it.