What is Open source
Software whose code is public and free for anyone to use, read, and improve. Most of it lives on GitHub.
In plain terms
Open source projects accept contributions from anyone, and the maintainers review those contributions in public. Developers contribute to learn, to build a reputation, and to fix tools they depend on. All of that activity is visible, dated, and attributable to a person.
What it tells you about a developer
Sustained open source work is unusually strong evidence, because it is voluntary, public, and reviewed by strangers. A developer who runs an open source project handles users, bug reports, and criticism in the open. That previews how they operate on a team.