This quote uses two distinct language techniques to illustrate the point.
The first of these is antithesis. This is a rhetorical device in which the author or speaker sets up an extreme contrast in order to emphasize a particular point (e.g., “highest of highs and lowest of lows”).
The antithesis in this passage is the juxtaposition of “Supreme Court of the United States” and “humblest J.P. court in the land.” While the Supreme Court is the highest court in the country, a justice of the peace court is considered to be on the lowest level of the judicial system. This contrast emphasizes the point that, regardless of the their relative "level" in the justice system, all courts should interpret laws justly.
The other language technique used in this quote is metaphor. A metaphor is a direct comparison without the use of the words "like" or "as." The metaphor in this quote is when the courts are called “the great levelers.” This metaphor is intended to underscore how courts are impartial and how, regardless of those involved in a case, the courts should remain objective. This metaphor, of course, is in reference to not allowing racism to impact the outcome of Tom Robinson’s trial.