Its a QOTO feature and yes you can access it from our web client.
It lets you follow a user without them knowing you are following them and without them needing to approve of you as a follower if it is locked down.
The down side is you will only see their public posts and any follower-only posts you cant see unnless you follow.
@herid I haven't seen subscribe, only follow. I wonder if it's something that comes up in web client? @freemo is the instance owner so may know.