Giving a star to an open source library means to a lot to the developers and consumers of the library. Because this will give the developers a positive impact to make the library better. It is definitely a way to tell gently to the developers, there are people out there who use that library.

As a consumer of the library

If you are using an open source library, go ahead and find its Github/Gitlab (or any version control source) and give it a star. If you need to use a high-quality generic library, you should help to bring new people to the library. Starring will bring quality and aliveness to the library. One of the people that you bring, might be the next contributor to the library.

To support the work or idea

You find an open source library in the following interests:

• Like the idea
• Plan to use it in your current or next project
• Like the software design of the library
• A topic of interests like distributed systems, data structures, useful links

Don't forget that any positive starring activity will bring more people to like the library with the social impact. This positive impact will also bring more contributors to the library. And will prevent the library to get out-dated.

These are only my ideas while starring a library, please do not hesitate to share yours. I will be happy to include in the story including your name.