Full form of RuBP is Ribulose 1, 5 biphosphate. RuBP is the first acceptor of atmospheric CO2 during dark reaction of photosynthesis. The reaction is called carboxylation.
6RuBP + 6CO2 + 6CH2O → Unstable 6C compound → 3PGA
RuBP is regenerated during final formation of sugar molecule.