In the philosophy of science, falsifiability or refutability is a quality or characteristic of a scientific hypothesis or theory. Falsifiability is considered a positive (and often essential) quality of a hypothesis because it means that the hypothesis is testable by empirical experiment and thus conforms to the standards of scientific method. That something is "falsifiable" does not mean it is false, rather it means that if it is false, then observation or experiment will at some point demonstrate its falsehood.
For example, the assertion that "all swans are white" is falsifiable, because it is logically possible that a swan can be found that is not white. Not all statements that are falsifiable in principle are falsifiable in practice. For example, "It will be raining here in one million years" is theoretically falsifiable, but not practically so.
The concept first popularized by Karl Popper, who, in his philosophical criticism of the popular positivist view of the scientific method, concluded that a hypothesis, proposition, or theory talks about the observable only if it is falsifiable. "Falsifiable" is often taken to loosely mean "testable." An adage states it loosely as "if it's not falsifiable, then it's not scientific". But the state of being falsifiable or scientific says nothing about its truth, soundness or validity, for example the unfalsifiable statement "That sunset is beautiful."
Falsifiability is the logical possibility that a proposition can be shown to be false by a single observation or experiment.
Take the Freudian proposition: All men have a subconscious desire to have sex with their mothers.
What could possibly show that to be false? Since it's about the subconscious nobody can falsify it by saying "no I don't."
"You don't think you do, but you're mistaken" will be the reply.
In general, the notion of a subconscious is a tumbling ground for whimsies. No falsifiability in any of it.
On the other hand, the law of gravity is in principle falsifiable. If someone perfects new measuring devices next year, they might prove that objects fall in a vacuum somewhat more rapidly than Newton's law says they should. So the law, as stated, would have been falsified.
In Popper's view, this means simply that Freudianism is not a science, and physics is.