What is real party in interest?

Section 2, Rule 3 of the 1997 Rules of Civil Procedure require (sic) that every action must be prosecuted and defended in the name of the real party in interest. A "real party in interest" is one who stands to be benefited or injured by the judgment in the suit, or the party entitled to the avails of the suit. "Interest" within the meaning of the rule means material interest, an interest in issue and to be affected by the decree, as distinguished from mere interest in the question involved, or a mere incidental interest.