In Guns, Germs, and Steel, what does plant domestication mean?
Plant domestication is the process of changing plants to make them more useful to human beings and then purposely raising them as crops (rather than simply gathering their fruit or seeds wherever they happen to be found).
Domesticated plants are physically different from their wild ancestors and relatives. They tend to have larger fruits and seeds, for example. This has come about through a process in which people select the plants that have the biggest fruits or seeds. The people plant those exclusively and eventually the typical fruit or seed becomes much bigger than the original.
This process of changing the plant to make it more useful is part of plant domestication. The other part is actually cultivating the plant rather than simply searching for it and gathering its fruit.
This answer can be found on the first page or two of Chapter 7.