Open Source Software (alias 'OSS') is a software where its source code are made available by the author for the users and often to the public as well. This has several major advantages, especially if you consider further tailoring the software for your needs or want to know (or at least be able to learn) what exactly the code you are using is doing.
Please note, that open source does not neccessarily mean free, although very often it is the case. However, there exist many different license types, where some of those may be almost as restrictive as for a closed source software and some have almost no restrictions at all.
Check also our extensive comparison of open source cons and pros.
