What is Pump Cavitation?

When the pressure of the suction line drops such that it is lesser than the vapor pressure of the fluid flowing inside the suction line, that causes to form vapor bubbles, which in turn collapses at the eye of the impeller results into cavitation.

(Besides them, another person was drinking a soda water from a bottle. He saw him and added)

To give you a simple example, the gas in the container is pressurized, and hence, nothing happens until it remains closed, but the moment you open the cap of a bottle what happens?

The bottle is suddenly open to the atmospheric pressure consequentially you observe that the entrained carbon dioxide starts releasing, this is because the atmospheric pressure is less than that of vapor pressure.