"Implausible" means "not seeming reasonable or probable", while "impossible" means "not able to occur, exist or be done," as you read in Oxford Dictionary of English.

Is it impossible or improbable?

As adjectives the difference between impossible and improbable. is that impossible is not possible, not able to be done while improbable is not likely to be true.

Does Impossible mean not possible?

not possible; unable to be, exist, happen, etc. unable to be done, performed, effected, etc.: an impossible assignment. incapable of being true, as a rumor. not to be done, endured, etc., with any degree of reason or propriety: an impossible situation.