Carbon dioxide (co2) combines with a five-carbon compoundin one to three sentences, describe what happens during the reduction stage of the calvin cycle.

We are well aware that there are two stages of photosynthesis:

Calvin cycle or light-independent reactions:

This comprise the process during which carbon dioxide enters into the leaves of plants and passes through series of steps to form sugar or food. This process depends on the supply of ATP, (the ATP that is formed previously during light dependent reactions).

Now there are three stages of Calvin cycle:

During the process of carbon fixation CO2 combines with a 5-carbon compound called RuBP or ribulose-1,5-bisphosphate which results in the synthesis of a 6-carbon compound that splits up in to 2 three carbon compounds called phosphoglyceric acid (3-PGA).

Here out focus will be the process of Reduction.

Reduction is the second stage of Calvin cycle during which phosphoglyceric acid (3-PGA) is converted to glyceraldehyde-3-phosphate (G3P) that is a sugar. During the process of reduction, energy in the form of ATP and NADPH  are used for the conversion of 3-PGA to G3P. As 3-PGA is reduced to G3P, therefore this process is known as reduction.

After reduction, a series of reactions occur that lead to the synthesis of glucose but since focus of our question was reduction, so you can see more details of the process in attached figure.

Hope it helps!