In science, constants are values that do not change. Even though constants can be measured, they cannot be changed during the course of a particular experiment.
To illustrate the idea of experimental constants in simple terms, chemists might study how two chemicals react at high and low temperatures while ensuring that the pressure does not change. If the pressure was allowed to change along with the temperature, it would be difficult to ascertain whether any observed differences in chemical reactivity were the result of pressure differences or temperature differences.
There are also physical constants such as the speed of light. Unlike experimental constants, there is no way to change the value of a physical constant.