This quotation is taken from act 2, scene 1 of Pericles, Prince of Tyre, one of Shakespeare's less well-known plays. It is spoken by the Second Fisherman to the titular character, Prince Pericles. The fisherman is pulling up his net when he becomes aware that something very heavy is stuck in the net. He therefore calls to his master to help him, presumably with lifting the unexpectedly heavy burden. In the dialogue quoted here, he is telling Pericles that what he believes to be a heavy fish is caught in his net.
However, he goes on to make some subtle observations about class structures and structural power imbalances. It is notable that this dialogue is spoken by a nameless fisherman, a lower class character. He tells Pericles that the fish is lodged in the net "like a poor man's right in law": that is, it is very difficult to get out. This could be interpreted in one of two ways, both with essentially similar meanings. Either the fisherman is saying that it is difficult for a poor man to actually access the rights that should be available to him through the law (meaning that the legal system favors the rich) or he is saying that the fish is stuck in the same way that a poor man might be stuck in the legal system in prison (again, meaning that the legal system favors the rich).
Ultimately, once the net has been pulled up, the fisherman notices that this is not a fish at all, but a piece of armor.