Generally speaking free software and open source are seen as the same thing. The reason is because both are free to download and for the most part free to use with only minor stipulations, if any. The reality is that the free software movement and the open source movement are a bit different.
Free software is software that is free in every sense of the word. Many languages have two words for free. One meaning that it is free as in no cost. The other meaning it’s free as in free to use and do with as you please. This movement holds both of these as being true. When you download a piece of software that is free, it doesn’t cost anything and it’s free to study, change, and use. The only catch is that if you make improvements it’s suggested you share those improvements with others.
Open source really is just a development model. You can have programs that do cost money and are not free to change or use as you please. A company could start a project to get people to join the community and help code, but they do not have to allow that project or software package to be free. Typically these projects end up being the same thing as free software but at times they aren’t. There is no official guidelines for what “open source” is so it’s important that you don’t accidentally distribute a piece of open source software that was not supposed to be distributed to others.