What part don't you understand?
"Imagined as only Orwell could ..." No one else could have imagined this story the same way that Orwell did. That is, Orwell, was very creative and original in coming up with this story, or this way to tell such a story.
"... this powerful fable ..." A "fable" is a story with some kind of message or moral to it. So the writer is saying that "1984" is a story with a message. A statement that I'm sure that anyone who has read the book would agree with. And it's not some weak, silly fable, it's a "powerful fable".
"... is instilled with humour ..." It includes amusing incidents. It's been a while since I read 1984, I don't recall it having any humor, but maybe I'm just forgetting.
"... and an underlying urgency that makes this one of the most prescient warnings ever given." The story is "prescient", that is, it predicts something that will or might happen. It's a "warning". It's not predicting something good; it's predicting something bad. And it has "urgency". It's not warning of some danger that might happen hundreds of years from now; it's something that might happen very soon.