The narrator of the poem is telling his friend to stop reading books; he'll become fat. The speaker then asks why he chooses to be so serious while outside there is a beautiful evening scene:
“Up! up! my Friend, and quit your books;
Or surely you'll grow double:
Up! up! my friend, and clear your looks,
Why all this toil and trouble? “
The narrator continues, telling his friend that books are dull and tedious. He says that instead of reading, he should go outside to where the “linnet (a small finch) and the throstle (a song bird)” are singing beautiful music which contains more wisdom than any book. Lines 15 and 16 are probably the most important lines of the poem: "Come forth into the light of things, / Let Nature is your teacher." The narrator is telling his friend that Nature has more to teach than books, and that he should go outside rather than seek refuge in dry pages.