The combustion process of igniting the fuel air charge is designed to expand in a certain way so that the whole charge is spent. It creates a 'wave' of flame as it expands. Engineers and fluid computational dynamics are used to design a more efficient shaped chamber, allowing ever higher efficiency of power vs consumption. Sometimes you get less efficient consumption of the charge. If some of the charge is unspent it explodes out of sequence and creates a shock wave that is so powerful that you can 'hear' it ringing in the engine structure. That is why a 'knock' sensor is really just a glorified microphone.
It is a precurser to pre-ignition, (which is different).

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