Can someone tell me what retopo is and what is it used for?
it is used to recreate the topology of your model- so you could use it for example if you had a face and wanted to create better polyflow, but a major use is normal mapping- create a hi res character with wrinkles and the works, you then create a normal map from that, and using the retopo, you trace out the polys over the hires character, and create a low res version of the same model that you can use in games or whatever, with a normal map applied to it to make it look hi res.