"Certain wavelengths" refers to specific frequencies or lengths of waves. It indicates that only waves with those particular measurements are being referred to or discussed. Full definition
It is a fact that CO2 only absorbs certain wavelengths of energy. (globalclimatescam.com)
No one really disagrees that, all other effects held constant in a laboratory, CO2 will absorb certain wavelengths of reflected sunlight. (climate-skeptic.com)
These materials can be engineered to enhance or suppress light reflection in certain wavelengths. (sciencedaily.com)