The speaker spends pretty much all of "A Supermarket in California" talking to Walt Whitman, the long-dead super important American poet. Why? Well, Whitman was one of Ginsberg's most influential inspirations, both in terms of poetic form and poetic content. Ginsberg's poems often look a whole lot like Whitman's do (sprawling poems with long sprawling lines), and Ginsberg writes a whole lot about Whitman's favorite theme: the good ol' U. S. of A. Some critics argue that "Supermarket" is all about Ginsberg trying to make a connection between himself and Whitman, and that Ginsberg is trying to cast himself as Whitman's literary son or heir. Shmoop think these critics are right on the money.
"Supermarket" successfully argues that Ginsberg is Whitman's poetic heir.
"Supermarket" is all about Ginsberg's desire to be Whitman's heir, but ultimately, it turns out that he doesn't live up to Whitman's amazing work. Sorry, buddy.