It shows up in the tab when France owns the province and can be taken when France also have invented Mission to Civilize. In practice it is impossible to colonize the province without having invented Mission to Civilize, so in reality the only requirement is to own the province.
It gives France 5 prestige and renames the province to Brazzaville. There is no way to rename the province back to Nkuna once it has been renamed.
Ingame text The Franco-Italian Adventurer Pierre Savognan de Brazza explored the Congo region in 1870's and 80's, and opened up the central Africa and the right bank of the Congo river to French colonization. The City of Brazzaville in French Congo was named after him.