Break point chlorination means:
The addition of chlorine to water to a point at which free residual chlorine begins to appear is called breakpoint chlorination.
The "chlorine demand" of the water should be estimated. The chlorine demand of the water is the difference between the amount of chlorine added to the water and the amount of residual chlorine remaining at the end of a specific period of contact at a given temperature and pH of the water.
The point at which the chlorine demand of the water is met is called the "breakpoint". If further chlorine is added beyond the breakpoint, free chlorine begins to appear in the water.

Essentials of preventive and community dentistry
Soben Peter
5th edition
Page number 466
Start of chlorination process
End of chlorination process
When free residual chlorine starts appearing
After partial saturation of water with chlorine