You can look at it in two ways. The pool player is an artist. He stands on the shoulders of giants: "...dead players; living critics/Are watching him" (line 6). When he plays, he is thinking not just of this one game but all the games that have been played before, and the techniques of the masters. He has to practice his game in slow silence until he is a master. It is the same with poets. They have to practice in slow silence for years, and always thinking of the poets that came before them--the dead masters.