The way I understand it the second thing is the observer of the organism, the person posing the question. The definition seems to be sort of equivalent to the statement "an entity is conscious IFF the sentence 'what is it like to be that entity' is well-posed".
"What is it like to be a rock" => no thing satisfies that answer => a rock does not have unconscious mental states
"What is it like to be a bat" => the subjective experience of a bat is what it is like => a bat has conscious mental states
Basically it seems like a roundabout way of equating "the existence of subjective experience" with "the existence of consciousness"
edit: one of the criticism papers that the wiki cites also provides a nice exploration of the usage of the word "like" in the definition, which you might be interested to read (http://www.phps.at/texte/HackerP1.pdf)
> It is important to note that the phrase 'there is something which it is like for a subject to have
experience E' does not indicate a comparison. Nagel does not claim that to have a given conscious
experience resembles something (e.g. some other experience), but rather that there is something
which it is like for the subject to have it, i.e. 'what it is like' is intended to signify 'how it is for the
subject himself'.