Does 'де ти' mean something like 'how's it going' or is literally asking 'where are you'?
Just "Where are you?"
Now that people often use mobile phones to talk to each other, and sometimes are late (or just cannot find the place they are suppposed to meet at)—that is quite a useful question to know :).
It just means Where are you?
Just a question: can you also say, "Ти де" as an infirmal question?
Yep, of course.
Hello, where you ?