If you want to answer this question, simply look at Figure 10.1 -- the map showing the major axes of all the continents. You will notice that the major axis of the Americas runs north and south. This is in contrast to the major axis of Eurasia, which runs east and west. Diamond argues that this is the major factor that made it difficult for crops and animals to spread in the Americas.
If a continent's major axis is east-west, much more of the land mass lies along the same latitude. This means that it will have more or less the same weather and more or less the same plants and animals can thrive all along that axis. That way, plants that are domesticated in one place can spread along that long axis.
But with the axis of the Americas, this is not possible. That axis covers lots of different latitudes from the equator up to much higher latitudes. A crop that was discovered in Mesoamerica would not be able to spread up and be raised, for example, in what is now Kansas.
The Americas also had other problems (deserts, thick jungles), but the north-south axis was the main problem. This is why Diamond even mentions the term in the title of the chapter.