The Falsification Principle was proposed by scientific philosopher Karl Popper. It proposes that for something to be scientific it must be be able to be proven false. If things are falsifiable (able to possibly be proven false) then they can be used in scientific studies and inquiry.
An example of a falsifiable statement is that all cars are red. This statement can be proven false easily with any observation of a car that is not red. Anything that cannot be proven false is considered pseudo-science and not valid for scientific inquest. An example of a unfalsifiable statement is that invisible trolls that cannot be detected by humans live inside trees. This statement cannot be proven to be correct or false. Therefore it is not falsifiable and cannot be used in scientific inquiry.