An airlock can be:
a) The system of two hatches used in space vessels, submarine escape compartments, decompression chambers or isolation units to prevent loss of pressure or contamination.
b) Imagine a liquid piping system which goes up then down. If air becomes trapped at the top it may take more pressure than available in, say, a drainage system or LP fuel system to move it on. Could, as previously stated be vapour lock in fuel system.
c) Some pumps designed for liquids, e.g: centrifugal pumps or diesel injector pumps are not very good at pumping gases and will cease to function thereby airlocking the system until primed with liquid.
I'm sure you knew all that already but use a different term for it.
I'd normally refer to the engine noise as a surge - good bang innit! - but tricky to explain to the passengers
