It's a pun on a way to explain open source: Free as in freedom, not free as in free beer.

http://en.wikipedia.org/wiki/Free_as_in_Freedom:_Richard_Sta...

So Free Beer isn't beer you don't pay for but it's recipe is freely available, i.e. open source.

Now you will also have to tell people that free software is free in the sense of freedom, and free in the sense of FREE BEER, and often but not necessarily in the sense of free beer.