This is white noise. It usually means the Luminance and all video circuits are working. This is typically going to be caused by an absence of signal or the front end (R.F. Amp) is not working. It also means the local oscillator and mixer stage in the tuner are operating. The tuner may not change channels. The presence of this random also gives the technician an indication that the I.F. and luminance circuitry are also functioning.
Having this type of symptom gives the tech a lot of information about the circuits that are working.