I would bet this is a term that most people have never heard.  I would go even further and say many people pouring concrete today are among that group.  Air-entrained concrete has millions of tiny air bubbles per cubic foot that helps alleviate internal pressures mainly due to expanding water as it freezes.  These air pockets allow a space for the expansion to take place without placing all of the expanding force directly against the concrete.  This extends the longevity of the concrete in the northern climates that go through many freeze/thaw cycles.