What is cavitation and explain the stage before cavitation happens?

For the first part of your question, Cavitation corresponds to vapor bubble formation, without raising the temperature but by a mechanical action. In the liquid, under the action of some agitation, such as movement of a helix, occurs locally sufficient pressure drop to cause their vaporization, even below their boiling point.

For the second part, I didn´t understand what do you mean by explain the stage before cavitation happens?