Free software is all about software that is free as in freedom, not as in gratis.
This is software that is licensed in a way that is friendly towards collaboration, openness and sharing. It’s software that gives the users the freedom to run the software for any purpose, as well as to study it, change it, and distribute it and any adapted versions. Because of this the source code must be available.
There are four freedoms that is important in the free software movement, and those are about the freedom to 1) run, 2), study, 3) redistribute and 4) improve the program/software.