what basically is purified here?

water is extracted from human excretory  wastes??

or what is the initial product ?

I think you are asking about the treatment of sewage, if so, here is the answer- 

As you know, human excreta (stools/feaces) alongwith water forms the main component of sewage which is disposed into rivers and other water bodies . This sewage contains harmful microbes which can be proved dangerous for both aquatic organisms & humans. Hence, this sewage is first treated in treatment plant in order to remove harmful organic matter and bacteria then clarified water (free from organic matter & microbes) is disposed off in water bodies. 

Hope it is clear to you now !