I think it's an expression of friendliness and at some point approval of their comments or behaviour. What I like about it on here as opposed general social media is that here it seems genuine, if someone follows you here it seems more considered. I don't know anyone going out of their way to be followed, unlike the norm these days and that's reassuring in a way...
In real terms it populates your personal news feed with content from followed people - What's new - Your news feed. So, if someone follows you, they can see what you've posted if they go to their news feed.
I suppose it means that if there's someone that we rate their posts etc we get to keep up on what they're saying with shortcuts to the content.