This is an example of decorative marginalia, which is quite common on medieval manuscripts. Sometimes the marginalia relates to the context of the subject of that page of the manuscript, but often it appears to have been quite random.
One fairly well-known group that I'm personally particularly fond of is the so-called animals at war which includes images like this:
- Breviary of Renaud and Marguerite de Bar, British Library, Yates Thompson MS 8, f. 294r
Another famous example is the nun picking penises from a phallus tree in the Roman de la Rose manuscript owned by the Bibliothèque Nationale de France in Paris (MS. Fr. 25526, f. 106v). (Available to view as a digitised document on the Bibliothèque Nationale de France's BNF Gallica website)
The Snail Combat Motif
In general, the meanings that should be attributed to the images that appear in marginalia is unclear, and you will find volumes of speculation on the subject. However, for the specific group you are interested in, the paper The Snail in Gothic Marginal Warfare by Lilian Randall may be helpful.
This motif emerged in Northern France in the late 13th century, and spread from there to English and Flemish marginalia. Lilian Randall's paper suggests a range of possibilities for interpreting the designs.
These interpretations range from the idea of simply fighting the snail as a pest (considering the damage that snails could do to vineyards), to linking the snail with a nickname given to the Lombards (who were frequently disparaged in the early Middle Ages).
She even notes a possible connection a modern version of the Mother Goose rhyme:
"Four-and-twenty tailors went out to kill a snail".
It is clear that Lilian Randall's own preference is for the link with the Lombards. She states:
From the assembled evidence the three questions regarding the origin of the marginal illustration can now be answered as follows: the predilection for the literary snail combat theme can be explained by the manifest current anti-Lombard sentiment; the rapid diffusion of the motif reflects the international character of the Lombards' professional activities; and finally, the concentration of the motif in late thirteenth- and early fourteenth- century manuscripts mirrors the intense reaction to a current development which gradually lost its appeal along with its novelty.
However, it is important to note that she she also concedes that the images could have had multiple meanings in different times and places.