What Rama agrees to do for Sugriva is kill Vali and reclaim the Kiskintha kingdom and make Sugriva the king.
Rama, or the Avatar of Vishnu, lost his wife Sita to the demon Ravana, the king of Rakshasas. He wants to bring her back, but in order to do that, he needs Sugriva's help. This is why he promised him that he would make him king in return for his help.
