FOSS means Free and Open Source Software.
It doesn’t mean software is free of cost. It means that source code of the software is open for all and anyone is free to use, study and modify the code. This principle allows other people to contribute to the development and improvement of a software like a community.
Freedom means having control over your own life. If you use a program to carry out activities in your life, your freedom depends on your having control over the program. You deserve to have control over the programs you use, and all the more so when you use them for something important in your life.
A free program allows you to tinker with it to make it do what you want (or cease to do something you dislike). Tinkering with software may sound ridiculous if you are accustomed to proprietary software as a sealed box, but in the Free World it’s a common thing to do, and a good way to learn programming. Even the traditional American pastime of tinkering with cars is obstructed because cars now contain nonfree software.