Whoever owns a U.S. patent has the power to exclude others (in the U.S.) from making, selling, using, practicing, distributing, etc. anything that is within the protected scope of the patent.  Then the question is, and has always been -- what is the protected scope?  A patent's protected scope is defined by the claim(s) listed in the patent.  So the claim(s) must be clear enough so that others can ascertain whether or not what he/she does would infringe the patent.

When a patent is in dispute, the person/entity accused of patent infringement usually argues that the claim language is too vague so that no reasonable person would understand what the claimed invention is, i.e., indefinite.  Hence, the claim is invalid because it fails to meet the patent law requirement that a claim must distinctly point out what is claimed, in other words, clearly defining the protected scope.

In the past, it is improbable, if not impossible, to have a court rule that a claim is "indefinite" because the court must find the claim to be "insolubly ambiguous."  Recently, the U.S. Supreme Court rejected the "insolubly ambiguous" standard as being overly permissive and fatally imprecise, and held that a claim is "indefinite" if the claim fails to inform, with reasonable clarity, those skilled in the art about the scope of the invention.  Nautilus, Inc. v. Biosig Instruments, Inc., 572 U.S. __ (2014).

It is interesting that the Supreme Court replaced one ambiguous standard with another one of no less ambiguity.  Understanbly, the Supreme Court did not want to set a bright line rule, as it is impractical to do so.  Thus, the only thing for sure now is "reasonable clarity" should be a lower standard than "insolubly ambiguous"; and an accused infringer should have a better chance on winning an indefiniteness argument.  However, it will take time to develop enough case law so we can better gauge this new "reasonable clarity" standard.  It is nonetheless a step forward so that patentees cannot resort to vague languages in order to endlessly expand the scope of their inventions.