> Nagel asserts that "an organism has conscious mental states if and only if there is something that it is like to be that organism—something it is like for the organism."

Struggling to make sense of this sentence.

An organism is conscious exactly when there is something it is like for that organism to be itself.

Or in a simpler way, consciousness is present just in case being that organism has an inner, subjective character - something that can not be reduced to a purely material state.

It means that if something has conscious mental states then it must have subjective experience from its own perspective. If John has a conscious mental state, then I must be able to ask "What is it like to be John?". Hope that helps.

I wouldn’t bother.