The poem "Jabberwocky" appears in Chapter 1 of Lewis Carroll's Through the Looking Glass and What Alice Found There. Alice finds the poem in a book and is unable to read it. She holds it up to a mirror and is able to read the words, but it doesn't help that much because most of the words are "hard to understand." Later she meets Humpty Dumpty, who elucidates the poem by defining some of the words. Carroll had actually written the poem when he was much younger, and he defined some of the words himself. Using a combination of Carroll's and Humpty Dumpty's definitions, one could understand the poem this way:
At four o'clock one afternoon, just when people start broiling things for dinner, the lithe and slimy badger-like creatures were gyrating and boring holes into the grass around the sundial. The shabby-looking birds were flimsy and miserable, and the lost green pigs were making a whistling bellow. The father warns his son to beware of the dangerous creature known as the Jabberwock, and also of the Jubjub bird and the Bandersnatch. The son goes forth with his sword seeking the creature. As he is resting by a tree, thinking in a rough, huffing temper, the Jabberwocky comes towards him through the woods. It makes puffing, bleating, and warbling sounds. The young man draws his vorpal sword and slices through the creature, killing it. He decapitates it and gallops home triumphantly. His father praises him, rejoicing with snorting chuckles. The final stanza repeats the first, indicating perhaps that life has returned to normal for the characters now that the manxome foe would no longer threaten them. (Manxome was one of the words Carroll left no explanation for; vorpal was another.)
Despite the poem's plentiful neologisms (made-up words), it is remarkably clear. Most people can understand the young man's courageous quest and the celebration at his success. Since English is a word-order language, readers can guess what parts of speech the nonsense words are, and their similarity to known words gives a sense of the meaning even before any definitions are given. The poem has been so popular that two of its nonsense words have made it into dictionaries as real words: galumphing and chortle.