Open source software is code or code packages that are developed under the General public License - GPL. 

 Under the GPL you are free to customize, modify and distribute all parts of the specific software (add-ons, templates, core, libraries, extensions etc.).

The GPL license is based on the principle that if you use GPL licensed software in your software or solution then this software or solution also has to have a GPL license.

Often the public refers to open source software as free software, hence referring to the price, but the focus is indeed in having your own freedom with your software. Having the source code, you can develop your application at any time, integrating it in other application modify the core or just customize the look and feel.

The rapid evolutionary process of open source software with its community and many independent programmers from different cultural backgrounds often produces better software than the traditional closed model, in which only a very few programmers can see the source and everybody else is bound by its limits .

With the wide range of possibilities comes a great responsibility for enterprises whether and how to use open source software (OSS), because OSS also needs support and maintenance, must be reliable and has to has to be properly installed or implemented. this support and maintenance is not part of the GPL. 

Support of OSS is given by its community which mostly organizes itself in projects, defining standards and creating documentations.