There are a couple different ways to explain it:
"Free as in freedom not as in beer"
or I kind of just came up with this one.
"You are free to study, modify, copy, and run the software any way you want"
with a quick contrast to a non-free program. Usually you can use something
that likely has malicious features. For example you could say as opposed to
skype which you can't study because you don't have the source code so you
have no idea if it spies on you. Even if it is gratis (don't pay for it) it
isn't free.
