The world serpent exists to be Thor's opposite number, and the sources stage the meeting three times. In the first, Thor goes fishing with the giant Hymir using an ox head for bait, hooks the serpent, and hauls it up until they are face to face. Hymir, terrified, cuts the line. The scene was popular enough to be carved on stones and it is one of the few episodes attested in both the poetry and the prose.
The second is a disguise. At the hall of Utgarda-Loki, Thor is challenged to lift a grey cat and manages only to raise one paw from the floor. Afterwards his host admits the cat was the serpent, that the company watched in horror as one end of the world came loose, and that Thor was tricked into a contest he could not be allowed to win. The passage is comedy that turns cold in its last lines.
The third is the end. At Ragnarok they kill each other: Thor strikes the serpent down, walks nine paces, and falls dead from its venom. The number nine recurs throughout the corpus and the count is deliberate. Taken together the three meetings make an argument that the poems never state directly, which is that the strongest of the gods and the thing that encircles the world are held in balance until they are not.