In a nutshell, this subscribe button allows you to decide what you see from friends' profiles. You can get all updates, only important updates or most updates. You can also decide whether you only want life events, photos or game notifications from people you know.
Big deal you say? Wait until the experimentation gets underway. For my purposes, I'm reducing the noise on my Facebook account big time. I'll probably only go with life events for most people. I care if you get divorced or have some big life change. You can spare me the details of your dinner and fun time in Jamaica. It'll also be interesting to see what Facebook's system deems important to you.
Granted, I'm becoming a more anti-social social networking guy since I've been systematically shooting things that take up too much time, but you can see where I'm heading here. Hmm to go unsubscribe or only the important stuff?
The other item of note from Facebook is it's allowing more folks to subscribe to you. If this sounds familiar, that's because the feature is out of the Google+ playbook. Again, Google+ has nice features, but Facebook will use its scale to popularize them.