Photorespiration is a process that takes place when CO2 levels in leaf cells are low. It reduces the efficiency of photosynthesis and the net productivity of the plant. Some plants have evolved alternate methods of photosynthesis that help to minimize losses to photorespiration.
The second phase of photosynthesis involves the dark or light-independent reactions. At the start of the light-independent reactions, carbon dioxide is added to a five-carbon sugar, creating a six-carbon sugar that will participate in a series of further reactions to form glucose. In photorespiration, by contrast, oxygen is added to the five-carbon sugar instead of carbon dioxide.
Photorespiration forms 3-phosphoglyceric acid and glycolate; the former is recycled through the Calvin cycle, whereas the latter is taken up by structures inside the cell called peroxisomes, which help break down the glycolate. Ultimately the process of photorespiration consumes oxygen and releases carbon dioxide. Since it reduces the efficiency of photosynthesis, it's largely counterproductive but nonetheless plays an important role in plant cells.
Some plants have evolved alternative photosynthetic processes that minimize photorespiration. Plants like corn, sugarcane and cacti are common in many environments thanks to these adaptations.