Software provided free of charge should not be on topic if its source is not available.
Software that is free in that it "gives users the freedom to run the software for any purpose as well as to study, modify, and distribute the original software and the adapted versions" should be on topic.
However, I'm not in favour of using the word "free" in the site description unless this distinction can be made very clear. Not everyone finding the site will happen to know this distinction, and we don't want to attract a lot of questions about software that happens to have no cost (or questions about versions of proprietary software that people have managed to obtain at no cost).
Finding a wording that happens to technically make the distinction clear won't necessarily avoid attracting questions from people who only read the word, rather than the sentence. We are going to have off topic questions no matter what we do, but I don't want a site description that makes this worse than it needs to be. I don't believe omitting the word "free" from the site description will prevent people with relevant questions from finding the site.
Anyone who knows what free software means will also know what open source software means. My guess is that the only people who will be put off using the site by the absence of the word "free" in the description are people who are looking for an argument rather than looking to ask a question. I would expect everyone who has an on topic question to find our site without the need for labeling it free.
As mattdm mentions in the comments, the help/on topic page should include clarification that free/libre software is on topic - that page is long enough to have a full and clear description of what is meant by free. People who click through to the on topic page are people who read carefully first. People who ask straight from the description page includes both people who read carefully and people who don't, so I don't want "free" to be presented in such a short format to those people who don't read carefully.