Falsifiability is a fundamental property of a statement of it being possible to have counter-examples to (the search for these counter-examples is called falsification). According to Karl Popper, claims that are not falsifiable are considered to be outside the realm of science. This criterion is accepted by most of the worldwide scientific community.
Special Creation, for instance, is non-falsifiable. Any theoretical circumstance is consistent with the idea of special creation. For instance, "God created the unicorns first, then erased them from existence, before coming up with the idea for Earth" is just as valid as "In the beginning, God created the heaven and the earth"
Evolutionary theory is falsifiable: To demonstrate that evolution is not true, one need only demonstrate that a certain trait could not form from successive mutations. This is known as "irreducible complexity", and despite claims to the contrary, it has never been effectively demonstrated, even at the molecular level.