I think I understand the clause itself, but I'd like to develop some intuitive understanding of why Marshall used the word "dormant." I read the quotes of his decisions where he introduced the word, and I just don't get it. Here is a definition of the concept:
Dormant commerce clause refers to a constitutional principle that is inferred from the commerce clause. The dormant commerce clause provides that the exclusive power granted to Congress through [the] commerce clause implies a negative consequence. The negative consequence is a restriction prohibiting a state from passing legislation that improperly discriminates against interstate commerce. Therefore, the dormant commerce clause limits the power [of] individual states to legislate on such matters. [The] Dormant commerce clause is not an express clause in the U.S. Constitution. It is, rather, a doctrine developed by the U.S. Supreme Court. The dormant clause doctrine is also known as negative commerce clause. USLegal.com.