Students of our textbook will recognize Nash equilibrium of strategic games from Chapter 15 and his non-strategic characterization of bargaining from Chapter 16.
Up until Nash's work on the theory of games, all we had were models of monopoly (one firm) and "perfect competition" (many small firms). This was not very useful because most of the interesting problems arise in oligopoly (competition between a few firms), and Professor Nash gave us a way to characterize competition in this setting.
Insights from his models have been distilled into advice given to generations of MBA students:
Use game theory to figure out where self interest is taking you; and if you don't like where that is, change the rules of the game.
Likewise his strategic and non strategic (axiomatic) models of bargaining have been distilled into bargaining insights:
The alternatives to agreement determine the terms of agreement.
John and Alicia Nash, RIP