"plus intéressant" would be "more interesting", but by adding the definite article, "le plus intéressant" becomes a superlative form, "the most interesting". http://french.about.com/od/grammar/a/superlatives.htm
thank you for the explanation... which makes good sense... however in English the article "the" would in some way require a subject... example.. this is the most interesting thing I have ever heard...in this example that Duo uses, the English translation is somehow missing something...You n6zs being a native speaker would understand what I mean....I think that is why you get translations where people try to figure out the French "le plus"
I think what is being implied here is that "It is the most interesting" is not a proper English sentence whereas "It is most interesting" certainly is. The former needs a bit of a preamble or "post-amble" to be correct. For example: "It's not the most elegant horse in the race, but it is the most interesting. Or, it is the most interesting item on the menu.
"C'est" only refers to a person if it precedes a modified noun. "C'est un homme" ("un" modifies "homme")