Suppose you're a student reading a physics book which explains the Theory of Relativity. Now, you are able to read the book, use the notorious formula E=mc^2 to solve all of your exercises and, if you're a particularly brilliant student, why not, even start from there to come up with a new scientific discovery. In other words, scientific knowledge is free for everybody to use, modify and redistribute -- you don't have to pay a royalty to Einstein's nephew every time you solve a difficult physics exercise or daydream about time-space travel.
The world of technology, on the other hand, is largely dominated by patents, which makes it a "close" and competitive model where knowledge is rarely shared without money involved: companies strive to patent their latest innovations and boost their profits. Standard licenses and patents therefore prohibit any attempt at understanding the detailed mechanism through which a particular piece of technology works.
In this sense, the Open Source Software can be seen as a way to bring the worlds of science and technology closer together, particularly in the field of computer software, by granting everybody the rights to disassemble, rebuild, manipulate and personalize the software, making it possible to understand its inner mechanisms and adapt it to the user's needs.
Many Open Source licenses therefore state the source code of the program must be available for immediate download. This, unluckily, is sometimes considered to impact the fields in which the OS model can be applied.
For instance, using free software for network security purposes is generally discouraged, as the source code availability makes it easier for a malicious user to find a bug in the program and exploit it, whereas with proprietary software you would have to reverse-engineer a large part of the code to achieve the same result.
However, the advantages in terms of transparency and reusability coming from the Open Source model are unique and will often attract developers willing to review and submit frequent patches and updates on a voluntary basis to ensure the quality of the project remains high.
