Google+ Sign-In recently and you aren't exactly sure what it does, then you've come to the right place. Google+ Sign-In is a new feature of the Google+ platform that extends Google to other web services. What does this mean? Well, now you can sign into other websites with you Google+ account. There's no need to transfer information to the new account because everything is already on your Google+ account, and your data stays safe. All you have to do is accept the permissions that are presented to you.
So what does doing this allow?
For the first thing, Google makes the experience more constant by prompting you to download the mobile app to your phone for the website you are now signed into.
After this, sharing becomes simplified. You can choose circles and individuals who can see the updates from this app, and whatever you do share can be clicked on the other individual to be taken directly where you were. The updates do not go directly to the stream (unless you explicitly share it), but rather are accessible from your profile.