A proposition or theory is '''falsifiable''' if it is possible to conduct , or imagine, an experiment such that, if could have an outcome at odds with the proposition were theory. This is not the same as actually being false, then it can be proven to be false.<ref>[http://www.encyclopedia.com/doc/1O87-falsifiability.html Definition] A Dictionary of Psychology, Andrew M. Colman, via encyclopedia.com</ref> A proposal or theory that could not possibly be shown to be false, even if it were false, is not falsifiable. Of course, if one can conduct an experiment and it has an outcome that is at odds with the theory, then that theory is both falsifiable and demonstrably false. The interesting cases of this are propositions that are believed to be true, and can be "verified" by experiments, but the experiments in fact have no way of having a negative outcome. This renders the experiments useless and the proposition unfalsifiable.
As an incredibly simple example, it is a generally accepted scientific principle that dropped objects fall downward. (Ignore various issues about imparting initial upward velocity, that is, throwing the object, or unusual frames of reference, like centrifuges.) But it is possible to visualize what it would look like if one dropped an object and it fell upward. Therefore, the theory that dropped objects fall downward is falsifiable.