Soundness

An argument is sound if, and only if, (1) the argument is valid and (2) all of its premises are true.

So suppose we have a sound argument:

All men are mortal.

Socrates is a man.

Therefore, Socrates is mortal.

In this case we have an argument where, first, if the premises are all true, then the conclusion must be true (i.e., the argument is valid); and, second, it so happens that the premises are all true. It follows that the conclusion must be true. That is the nice thing about soundness: if you know an argument is sound, then you know that the conclusion of the argument is true. By definition, all sound arguments have true conclusions. So soundness is a very good quality for an argument to have.