General
Open Source
Also written as OSS
Software whose source code is publicly available for anyone to use, inspect, and contribute to.
Think of it like
Like a recipe published in a public cookbook that anyone can cook from, tweak, or improve, instead of a restaurant's locked recipe binder.
Junior or senior?
Junior sounds like
Lists open-source projects they've only used.
Senior sounds like
Has public contributions you can actually look at the commits for.
Ask them
“Do you have any public open-source contributions I could look at?”