It means “Where are you?” But spoken with slang or incorrect grammar.

‘Where you at’ is a very informal slang way to say ‘where are you’. Usually used over text, it’s not usually used in a formal situation, because it’s viewed as unprofessional. But, if you’re talking to a friend casually, you can say ‘where you at?’ to ask their location! :)

It could also mean “What stage are you at with something” How far have you completed a task.