It is pretty simple to understand.
Free source in the sense: Which is free with free laws applicable once you modify it also should also be shared to world with same laws! And its code if any can also be all of us!
Open source in the sense: We can see the actual application for our daily usage, but cannot use it as it is ours means that we cannot modify its code since it is copyrighted to the distributor!
For more info: "Difference between free and open source"
