I find the following:
http://www.yoctoproject.org/docs/2.1/bitbake-user-manual/bitbake-user-manual.html#accessing-datastore-variables-using-python
It said that: Using "expand=True" expands the value. What does the "expand" means?
Great that you are reading BitBake user manual, it helps a lot in understanding the recipe syntax.
Returning to your question the 'expand' means that in case when this particular variable value depends on some other variables, e.g:
B = "architecture_${A}"
and A is equal to "x86", calling:
d.getVar("B", expand=True)
will return you: "architecture_x86" as the variable A has been expanded.
Some other examples can be found in BitBake User Manual Chapter 3.