Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Open source means different things to different people.
To some, it's simply that the software code is out there, available for anyone to download, change and re-release. Maybe it's free; maybe you have to pay for it; maybe the code is free, but support comes with a price.
To some, it's a concept. It's a philosophy. It's a way of developing and sharing materials.
Heck, just google "what is open source?" and you'll get a slew of different answers, from a strict dictionary definition, to wit: "Of, or relating to a product which is licensed to permit modifications and redistribution of its source code"; to a seven-part description from the Open Source Initiative, which focuses primarily on how something is licensed and what those licenses allow.
To some, the dictionary definition is all that's important. Can the source code be modified and redistributed? Yes? OK, good to go. That's open source. And to a degree, that's correct.