Ra is a prefix in many titles (ramasi, ramalo, rasau, ravunisa, ratu), and tu means simply "chief".
In Fijian, one would say, Gone Turaga Na (Styling) Roko Tui Bau (noble title), Ratu (address/title) Joni Madraiwiwi (name).
(He was only recognised by the British and a few provinces of Viti Levu) He along with 12 high Chiefs subsequently ceded the islands to the United Kingdom in 1874.
They established what was to become the Great Council of Chiefs, originally an advisory body, but it grew into a powerful constitutional institution.
Constitutionally, it functions as an electoral college to choose Fiji's president (a largely honorary position modelled on the British monarchy), the vice-president, and 14 of the 32 senators, members of Parliament's "upper house", which has a veto over most legislation.
The 18 other senators are appointed by the Prime Minister (9), the Leader of the Opposition (8), and the Council of Rotuma (1); these appointees may, or may not, be of chiefly rank also.
The presidency, vice-presidency, and fourteen senators are the only constitutional offices whose appointment is controlled by persons of chiefly rank.
In the years following independence, this favored the chiefly class, as the common people looked to them as their leaders and generally voted for them.