RuBP is the intermediate of the Calvin cycle onto which CO2 is added in the process of carbon dioxide fixation. The reaction below is catalyzed by the enzyme called rubisco

RuBP + CO2 + H2O <=> 2 3-Phosphoglycerate + 2 H⁺ 

RuBP is made by action of a kinase acting on ribulose-5-phosphate, as follows:

Ribulose-5-Phosphate + ATP <=> RuBP + ADP