Before the site used to have "friends" and a live chat feature, so naturally you'd add people to chat with them.. but then they took off the chat and the friends thing.. so when the site updated my 4 friends became people i follow.
I don't really use the follow feature. If someone follows me I'll follow them back, but I check on that very infrequently. So anyone who follows me that I'm not following, know that its nothing personal :-P
You go to their profile, and at the banner that hold their profile picture, follow it to the left and you'll see a "follow" button. & you know when you look at your own profile & under the "active summary" chart, there's a "followers" row, followed by the number you have.