From below example you can understand the what is validity of argument.
Validity is a property of the argument's form. It doesn't matter what
the premises and the conclusion actually say. It just matters whether the
argument has the right form. So, in particular, a valid argument need
not have true premises, nor need it have a true conclusion
All cats are reptiles.
Bugs Bunny is a cat.
So Bugs Bunny is a reptile.
Neither of the premises of this argument is true. Nor is the conclusion. But the premises are of such a form that if they were both true, then the conclusion would also have to
be true. Hence the argument is valid.
(b) argument is not valid
We can
test for invalidity by assuming that all the premises are true and seeing
whether it is still possible for the conclusion to be false.
so we can say that if argument is invalid then conclusion is always false.