Benjamin is, as the eNotes summary says, a cynic. He thinks that he knows better than all the other animals. They all have hopes and dreams. For example, they thought that their revolution would improve their lives. But Benjamin knows better. This is why he makes this remark about the "unalterable law."
What he is saying is that nothing is really going to get much better or much worse. Trying to improve things, in other word, is futile so you may as well not get too excited about things one way or the other.
Benjamin's views are said to be those of Orwell himself. This means Orwell is saying that revolutions and such cannot really change life much because human nature is set and makes life pretty unchangeable.
Benjamin is one of the oldest animals on the farm and is still alive in the concluding chapter of the novel. Consequently he is well qualified to comment on the final outcome of the entire history of the rebellion of the animals and the final state of affairs on the farm.
In the last chapter of the novel Orwell concludes that although the rebellion of the animals began very optimistically it has now deteriorated to a stage where the pigs - the ruling elite - had completely hijacked the rebellion for their own selfish ends, so much so now they are no different from the men whom they had overthrown:
The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
Benjamin, of course, is the intelligent cynic, the voice of Orwell who comes to the sad conclusion that the lot of the common man will never change, no matter the form of government, as long as man is greedy, selfish, and power crazy. And that is why he concludes that the lot of the common man is one of unmitigated misery:
hunger, hardship, and disappointment being, so he said, the unalterable law of life.