Subscribed was changed to follow when it comes to people (and soon brands). It makes more sense as most people assume you can just subscribe to threads, not brands, products or people. The functionality has been here for a long time, but adding the modules to your profile to see who you are following and who is following you was done in February
An example of how I use the feature is I subscribe (AKA follow) Robert Harris since I like reading his threads. Whenever he starts a new thread I am automatically subscribed to it and it shows up like any other subscribed thread in my activity feed. So under Public Messages he would see a heading that says Members Following You and he would see my name and avatar. To subscribe/follow someone you can hover over them name in the forum and select follow member from the pop-up menu, or do it from their profile page. Under Personal Messages in your profile page you will see everyone you are subscribed/following and everyone who is subscribed/following you.