I star repos when I want to bookmark them. I plan to use it in the future or am interested in reading through the source.
There are many factors, "stars" among the more important and obvious ones. It indicates that this project was popular at one time for one reason or another. There's not enough info in the "stars" alone to draw any more conclusions without looking at other factors (although I am biased and favor a project with a lot of stars without looking at any other factors).
Other factors that I will consider:
* How old is the last commit? (is this actively maintained)
* How many contributors? (have multiple developers reviewed and worked on this code base)
* How many open issues (vs how many closed issues)?
* When was the last issue resolved? (also actively maintained)
* Are there CI tests built in and are they passing?
* Is there test coverage reported and is it acceptable?
I'll also:
* read through a few issues, make sure no one is saying "This does not build" etc.
* scroll through some of the dependencies (badges can help by indicating if the dependencies are out of date).