Open source software is the opposite of commercial software. It is written by volunteers and is available for free. Except of course that there is still a cost to install it, and train people how to use it a and an ongoing cost for maintaining it. In general, open source products are not as easy to set up and use by unskilled staff and some of the money saved by not buying commercial products may be spent on additional support.
The more well-known open-source products such as Linux (operating system) Postfix (Email) and MySQL (database) are solid and reliable. Some products however can have an unfinished feel, especially when it comes to documentation and installation.