What does it mean to say an deductive argument is valid?
To say a deductive argument is "valid" means that if all of the premisses are true, then the conclusion must be true.  This may also be expressed by saying the truth of the premisses (all of them together) necessitates the truth of the conclusion.  Two more ways of saying the same thing are "The premisses imply the conclusion." or "The premisses entail the conclusion."

The concept of "validity" is the central concept of deductive logic.

Notice first of all that it refers to a property of arguments, not statements. Thus a premise or a conclusion is not said to be "valid" or "invalid," but "true" or "false."

Nor is validity to be identified with the property of an argument having a true conclusion. Valid arguments may have false conclusions. This is so because saying that the truth of the premises necessitates the truth of the conclusion amounts to saying that if the premises are true, then the conclusion cannot be otherwise than true. The emphasis here is on the "if"; no claim that the premises are in fact true is made in claiming that an argument is valid. All that is claimed is that the conclusion necessarily follows from the premises, or in other words that should the premises turn out to be all true, then the conclusion would have to be true.

Another way to put this is that it is impossible for a valid argument to have all true premises and a false conclusion. If it can be shown that an argument does have all true premises and a false conclusion, then, ipso facto it is invalid.