The combustion in an IC engine occurs when the piston is at the Top Dead Center (TDC) at the beginning of the power stroke.
The combustion causes the expansion of the gases which pushes the piston down.
Knocking occurs when the combustion occurs at during the compression stroke when the piston is still moving towards the top dead center. The power developed in such a situation is not useful. The power developed during knocking can damage the engine components. For instance, knocking may remove material from the walls of the cylinder. If knocking occurs for many cycles, the valves may get damaged.
Knocking
Knocking is usually accompanied with a pinging sound.