This is a companion feature to the thumbs-up.
The thumbs-up is a way to show your approval of something that you like.
The stars, on the other hand, are a more personalized way to bookmark something on the site that you want to follow or find later on.
If there's a post that has a tutorial that you want to come back to, you can star the post.
Then, from your profile, you can click on the number next to the word "Stars" and see all the items you have starred.
Most things that can be thumbed up can also be starred.
