Lentils are traditionally eaten in Italy on New Year's Eve to symbolize money and prosperity.
The association comes from the legumes' small, round, coin-like appearance. Eating lentils after midnight, often alongside cotechino or another pork sausage, expresses a wish for financial abundance in the coming year.
Italian New Year's Eve is known as La Festa di San Silvestro, referring to Saint Sylvester's feast day on December 31. Customs vary by region and household, but festive meals, sparkling wine, fireworks, and public celebrations are widespread.
Cotechino with lentils is a particularly famous combination, especially in northern Italian culinary traditions. The lentils are the prosperity symbol; the sausage is not itself the answer to the question. Similar money-related food symbolism appears in other countries, but the Italian New Year's pairing is especially strongly associated with lentils.