“Founder crops” are crops that become the basis of agriculture in a given region. They are the first crops to be domesticated. This allows agriculture to begin and, eventually, to diversify to where crops other than the founder crops are domesticated.
Founder crops were important in the Fertile Crescent because they allowed that region to get agriculture before any other region of the world. Because the Fertile Crescent got agriculture first, it also developed civilization first. This allowed Eurasian civilizations to become more powerful than those of any other region of the world.
The founder crops in the Fertile Crescent were particularly plentiful and particularly well-adapted to human use. On p. 139 in the paperback edition of the book, Diamond tells us that the majority of wild grasses with the largest seeds were native to the Fertile Crescent or nearby regions. What this meant is that people who lived in that area had a large number of plant species to pick from when they started to move towards farming.
Thus, the founder crops of the Fertile Crescent were important because they were plentiful and useful to humans, thus giving people in that region a head start towards civilization.