There's no perfect brief answer. In terms of the GPL2 (which is usually what
you hear about), it works somewhat like this:

I receive some software (X). With it, I get access to the source code
(whether it gets mailed to me as a CD, is available for download, etc).
While I am free to use the software unchanged, I am also able to make
changes to the software. I can repackage, redistribute, even sell my
resulting work; but I must pass along the same rights. The people I'm
selling to must be granted access to the same source that I use and work
with, and are to be allowed to create their own derivative works, etc.

There's more to it than this, and it varies with other licenses, but that's
the gist of it.

-- 

Jordan Peacock
hewhocutsdown at gmail.com
hewhocutsdown.blogspot.com
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