The noise generator is a module that creates noise, that’s the sound that is produced when you tune between the stations of a VHF /FM radio.
The sound is white noise and is the most common noise that a generator will create. You may also encounter pink noise which starts as white noise but has some of the higher frequencies filtered out.
There are other noise colours but most are not really that essential in electronic music applications as the raw white noise signal is usually filtered anyway to produce the sounds you want.
By filtering white noise you can produce all sorts of percussive and drum type sounds along with wind, rain, sea and thunder. Noise can also be mixed in with the outputs of oscillators and can be separately enveloped to add noise chiffs and effects to notes.