Watch
Following a repository so GitHub sends updates about it. Watching takes one click and requires writing no code.
What it tells you about a developer
A developer who watches a project is paying attention to it over time. That is a stronger read than a star alone, and the hands-on skill question is answered by their code.
Watching sits between a star and real engagement. A star is a one-time bookmark, while a watch means the person wants ongoing updates, a small but continuing act of attention. Most developers watch far fewer repositories than they star, since watching adds noise to their notifications.