Your digital tuner examines the incoming signal to try to separate what should really be there (i.e. what was broadcast by the transmitter) from extraneous noise (which can come from other radio signals on the same or adjacent frequencies, interference from electrical devices in the area or even from solar radiation caused by sunspots and solar flares).
The ratio of 'good' signals to 'rubbish' ones determines the 'quality' of the signal.
If you're Freeview signal occasionally breaks up (particularly on certain channels, or groups of channels) you need to improve your aerial system:
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Chris