"Open Source Software" refers to computer software in which a portion of all of the code that makes up the software is accessible for review and modification by anyone with the technical ability to do so.
Open source software is typically free although some companies will require a fee to gain access to the source and the licensee will be required to keep that source private.
The opposite of open source software is closed-source software.
With Closed Source Software typically has a fee or may be free. The code that makes up the software is hidden from view and requires some type of password or de-encryption process for access, if it's even accessible at all.