Literally, Dua means 'calling'. Therefore, remembering Allah and asking support from Him is dua. Rasulullah says, "Dua is Ibadah". So, we need to dua to Allah to accept our prayers and to demand Jannah in the Akhirah. Rabbana means 'Allah, Lord, Rob'. Duas, those are started with Rabbana, are called Rabbana Dua.