In Italian it translates as 'ecco'. In French 'voila' . So it's a casual way of saying 'here it is' when giving someone something (for example a waiter in a cafe serving you your food) , or showing someone something (for example showing a place on a map).

It is a form of casual encouragement. It equates to saying; "That's good".

Agatha and Bruce have explained the usual usage, but let me add that if it is followed by the word "again" it has a completely different meaning. "There you go again" means "you are again doing or saying something stupid or not helpful." Example: "I think we'll be able to double our business next year." "There you go again with one of your wild ideas."