A free lunch is a mythical meal whose raw materials and preparation cost nothing. There is a saying, popular among economists, that "there ain't no such thing as a free lunch" (TANSTAAFL). Even if one receives a lunch as a gift, the giver had to incur the costs of production. And even if one plucks fruit from a tree, a cost is still incurred in land (the tree) and labor (the effort requires to pluck the fruit).
Because it is common knowledge that there is no such thing as a free lunch, people are often inclined to view offers of "free" products with suspicion. Often, there is some sort of catch, such as having to listen to a sales pitch or fill out a questionnaire.