Open source
Software whose code is public and free for anyone to use, read, and improve. Most of it lives on GitHub.
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.
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.