Source: http://www.april.org/articles/intro/ll.html (in French)
"Image your are having diner at a restaurant and that you are eating an excellent dish. Maybe you would like to cook this meal the next day at your place for your friends?
It is impossible, because you don't have the recipe for the dish. You can still eat it at the restaurant, but even if you know the taste of it, you don't know how to reproduce it.
In IT, it's the same thing with software. Most of software are distributed without their recipe, and it's forbidden to try to understand their features (in this case we talked about proprietary software). Sharing with your friend or trying to modify it to fit your needs is not allowed.
On the other hand, a open source software guarantee the following rights:
The access to the source code is a condition for all of this.
WIth an open source software, you have the dish, the recipe and the right to redistribute (or sell) the dish, the recipe or you can also modify it.