Nagel's What is it like to be a bat? is a classic work, centred around the human consciousness. It was published in 1974 which was around the time that physicalist arguments, which would reduce the mental to the merely physical, were becoming a formidable front in the investigation of consciousness.
Perhaps the most essential point he makes within the work is that comments on an experience are by nature always subjective. The whole idea of an objective account therefore makes no sense - no more sense than asking what someone's inward experiences are really like, as opposed to how they seem to that person.
Nagel seeks to prove that the entire nature of the idea is so difficult to comprehend that to even capture the attention of an audience, he must make them think about the subject by asking them a "trick question". "What is it lke to be a bat?" is designed to make the reader think about what they believe or imagine it "would be like" to be a bat, while at the same time making them (hopefully) consider that they cannot ever actually know. If he were able to explain this theory straightforwardly, he would be making a contradiction of the theory itself.
Certain experiences that a bat undergoes are obviously impossible for a human to experience, at least first-hand. The echo-location sense of a bat, for example, means that a bat is critically different from a human being, enough so that Nagel's point is dramatised to a level that asks the question at a more philosophical level, rather than the physicalist level readers of the time may be tempted to answer in. Claims that a human could receive data such as the location of an object etc. from the sounds they receive fail to see the point of the concept Nagel is attempting to convey.
After considering What is it like to be a bat? , the reader gains a new appreciation of what it is to be conscious. A new light is shed on the argument regarding the boundary between physical and mental experiences, and further questions are ironically revealed.