What is Maintainer
A developer trusted to review other people's code and decide what gets accepted into a project many people rely on.
In plain terms
Maintainership is earned trust in the open source world. Maintainers sort bug reports, review submitted changes, and gate what ships into a project many people rely on. Nobody appoints themselves maintainer: the community extends that trust based on demonstrated judgment.
What it tells you about a developer
A maintainer has senior-shaped judgment by definition: they evaluate other people's code, say no diplomatically, and carry long-term responsibility. Maintainers also tend to interview candidates well, since reviewing strangers' code is their daily practice.