Ra is a prefix in many titles (Ramasi, Ramalo, Rasau, Ravunisa, Ratu), and Tu means simply "chief". The formal use of "Ratu" as a title in a name (as in "Sir" in British tradition) was not introduced until after the cession of 1874. Until then, a chief would be known only by his birth name and his area-specific traditional title.
Regional variations include Ro in Rewa and parts of Naitasiri and Tailevu. Roko in parts of Naitasiri, Rewa and Lau (particularly the Moala group), Ra in parts of Vanua Levu, particularly the province of Bua.
In all those places, it is used as a title preceding the person's name, much like "Prince", "Duke", "Earl", "Baron" or "Lord".
The semantics, however, are a little different in Fijian although the name and title are usually reversed, for example:
In English, one would say His Royal Highness (Styling) Prince (address/title) Andrew (name), Duke of York (noble title).