In a recent article posted on The Jousting Life found here, the article talks about the quantity of images that appear in medieval literature that depict a knight battling, sometimes jousting, a snail.
The context surrounding the illustrations is vague, causing historians to argue, as they do so well, over the true meaning of the illustrations.
After reading the article, and reading the opinions of said historians, the opinion that makes the most sense is that the illustrations most likely would represent a peasant uprising against the nobility.
A knight in shining armor is the key image of noble strength. Power, courage, skill; however in these pictures, the knight is almost always either losing or depicted as cowering from the snail. The snail is a dirt dwelling worm; a nasty, poor, lowly creature. However, it has armor all its own. This would be how a noble might look upon the snail; as nothing more than a disgusting pest. In the illustrations, the snail being large and menacing and overcoming the knight would show the artists feelings that if the "snails' or peasants want, they can overcome the nobles. And history has proven that peasants uprising can accomplish much against the nobility.
Please, go to the website and read this amazing article for yourself: