When Jerome and George admire the magnificent specimen of trout in the glass case, various locals come into the room, one by one, each one declaring that he caught it, individually. This obviously cannot be true and so it becomes clear that they are just trying to show off in order to impress Jerome and George. Jerome remarks earlier in the chapter that the trait of exaggeration and downright lying is common to many fishermen, observing humorously that he himself would not be able to compete:

I should never make anything of a fisherman. I had not got sufficient imagination. (chapter 17)

 Finally the landlord comes in and claims that he caught the fish, and that all the others have been lying. However, it is eventually revealed that the trout is just a model, and so the landlord is proved to be as deceitful as the rest.

The whole affair illustrates the human capacity for lying and boasting. The book is full of such light-hearted incidents that reveal truths about human nature in an amusing way.