There are two possible ways to understand this question. First, it may be asking what the definition of “founder crops” is in Chapter 5 of Guns, Germs, and Steel. Second, it could be asking which crops are listed as founder crops in that chapter. I will answer both of these questions.
Chapter 5 is discussing places in the world that did and did not develop agriculture on their own. In some places, people developed agriculture using plants and animals native to their own area. In other places, the first crops came from another place where they had first been domesticated. These first crops are the founder crops because they were the crops on which agriculture was “founded.” As Diamond says on p. 100,
imported domesticates may be thought of as "founder" crops and animals, because they founded local food production.
So what crops are listed as founder crops in Chapter 5? These crops can be found in Table 5.1 on p. 100. There were different founder crops in different places. The founder crops in Western Europe were poppies and oats. The founder crops in the Indus Valley were the eggplant and sesame. Finally, the founder crops in Egypt were the sycamore fig and chufa.