Understanding OSS & How They Are Used!

Just currently trying to wrap my brain around OSS and the many ways in which libraries use them or choose not to use them.  The moment I think I understand what Open Source Software is I find articles or blogs discussing how OSS goes beyond just being a software, it is community of open thoughts and shared information; can be free, but can also be costly if not used correctly.  There is so much data written on this topic and I am trying to let the information itself sink in at this point.