Fractionation is a measure of how different isotopes of the same element behave during chemical reactions.
Different isotopes of the same element all behave more-or-less the same way in chemical reactions. They all bond in the same places, to the same other atoms. They fit into the same spaces. However, because the different isotopes have slightly different masses (weight), there is a tiny difference in the strength of the bond when there’s a heavier isotope or a lighter isotope. This tiny difference results in the heavy isotopes being separated from the light isotopes in a process called fractionation.
The degree of fractionation is affected by a whole lot of things, including temperature, the rate of the reaction, and whether or not the system is in equilibrium. Telling these things apart is not always easy. In fact, sometimes it’s downright impossible.
The more we understand about fractionation, the more we can use isotopes to tell us about invisible and sometimes ancient processes that we could never understand otherwise. Fractionation is one of the tools we have to recognize the presence of life on Earth, even without fossils, because metabolic processes like simple respiration result in fractionation of carbon which then can be recorded in rock.