This story, structured in beautiful meter and excellent rhyme, is about how we set ourselves huge goals, and imagine huge success, only to put it off because we prefer to sit under a tree and pass our days catching the rays from the sun. The protagonist in this book (known only as 'I') is a man who has great hopes for his dog because he wants to teach his dog 100 words, but not only teach him those words but to also teach him how to do these things.
It seems that the first thing he wants to teach the dog is to do what we call 'busy work' which is work that has no appreciable outcome other than to keep people busy – such as digging a hole and then filling it in again. However he also wants to teach his dog very complex things, such as painting a road, cleaning a zoo, and giving a moose a clean with a mop. It seems that the protagonist believes that his dog is a very intelligent dog (but since he never actually does it we don't know how intelligent this dog really is: we can only speculate).
If he does succeed (and there is no evidence that he will, because he is only speculating) then he imagines that he will become the talk of the town because he has such an amazing dog that can spray paint chairs, perform marvellous feats of acrobatics, and clean a moose with a mop. As such he imagines fame, glory, and a public holiday dedicated to his dog.
However, despite all of these grand plans, in the end it seems that it is simply too hard, and the sun too nice, that it is better to go outside, lie down under a tree, and go to sleep.