When the code of software or the inputs are known, so you can audit it and see how it all works. Kinda like opening the hood of your car, but for software.
Opposite of Closed source
WeWrite used to be open source but once I began working a software job I closed source all of my projects so I could code during off-peak hours and not feel guilty (even tho I'm not technically on call, it's one teams message away from "just saw you push code" lmao)