Star
A public bookmark another GitHub user gives a repository, like a favorite. Star counts measure a project's audience.
What it tells you about a developer
Stars on repositories a developer authored are peer endorsement at scale. Low stars carry no negative reading, since most excellent working code has no audience.
Starring takes one click and works like an upvote. Most repositories have zero stars. A few hundred means a real audience found the project, and thousands means significant ecosystem attention. Stars track visibility and promotion, and they can be gamed or bought, so they measure reach.