Using “at” at the end of “where” questions

1. Where are you?

This construction usually refers to your physical location. It is grammatically correct.

2. Where are you at?

This construction is likely to refer to how far through a particular task you are, such as reading a book, travelling somewhere, or thinking about a problem. This construction is a colloquial idiom, and is not correct in formal English. It would usually be tautological and grammatically incorrect to use the preposition "at" with the word "where" but this particular construction ("Where are you at?") has gained currency in informal communication.