Free software is software (computer program) that anyone may run, share and change, at any time, for any reason. In this case, "free" means "freedom-respecting" (we say "free as in freedom"). The opposite of free software is proprietary software.
In 1984, Richard Stallman started the free software movement when he began the GNU project. Examples of free software are Linux (the kernel), Blender, OpenBSD, Inkscape and others. Wikipedia also uses free software.