A catalyzer, in chemistry, is a substance that helps too other substances to react. For example, consider the case of ordinary cane sugar and water. Dissolve the sugar in water and nothing happens. But add a few drops of acid and the sugar changes to glucose and fructose. Meanwhile, the acid itself is absolutely unchanged. All it does is to stir up the reaction between the water and the sugar. The process is called catalysis. The acid is a catalyzer.
Well, this is almost exactly the function of a genuine critic of the arts. It is his business to provoke the reaction between the work of art and the spectator. The spectator, untutored, stands unmoved; he sees the work of art, but it fails to make any intelligible impression on him; if he were spontaneously sensitive to it, there would be no need for criticism. But now comes the critic with his catalysis. He makes the work of art live for the spectator; he makes the spectator live for the work of art. Out of the process comes understanding, appreciation, intelligent enjoyment --- and that is precisely what the artist tried to produce.
from Prejudices: First Series, "Criticism of Criticism of Criticism" by H. L. Mencken