The passage's language is somewhat difficult, but it appears to speak of eating or dipping2 some food before the main meal. The Mishna mentions chazeret (lettuce) explicitly, but the truncated phrase "and they brought before him" allows for the possibility that other foods or vegetables were brought as well. No reason is given for the custom, and at first glance it would seem to have nothing to do with Pesach and the story of the Exodus. Why, then, has the custom been incorporated into the Seder?
First Course
In the Mishnaic period it was a common practice in Israel that festive meals began with a series of appetizers. This is attested to in several Tannaitic passages,3 which mention assorted appetizers as being part of "סדר הסעודה". Thus, for example, Tosefta Berakhot 4:8, mentions a series of three "פרפריות" that would be served to guests. As the source speaks of washing hands, it further seems that these were normally dipped in some sort of liquid (as only then would ritual hand-washing be required). Such festive meals likely served as the model for the Seder,4 and thus, in its earliest stages, the "vegetable dipping" of Karpas likely had no religious significance. It was simply the natural opening of the meal, meant to whet the appetite for later courses.5 Lettuce is probably mentioned explicitly, because it was the most common appetizer.6
Sign for the Children
In Amoraic literature, a different explanation of the custom appears. Bavli Pesachim 114a and 116a imply that the first dipping/eating was instituted only so that the children will ask: "כי היכי דליהוי היכירא לתינוקות".7 This new understanding stems from the different forms that meals took in Babylonia and Israel. The dipping of an appetizer was not common practice outside of Israel, so the Bavli did not see being "מְטַבֵּל בַּחֲזֶרֶת" as a normal part of the meal, and needed to explain its presence in the Seder. Thus, the gemara posits that the custom, like other unusual acts done at the seder, was instituted only to provoke questioning.
