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.