@b6hydra its not a mastodon feature, it is unique to QOTO.
Subscribe brings their posts to your home TL. But only posts you have permission to view, so public and unlisted posts only. It however does not require the persons approval and does not notify them when you subscribe.
Follow on the other hand may be locked down (require the persons approval), they are notified when you follow, and will also give you access to follower-only posts.
@b6hydra happy to help, hit me up anytime if your confused.
@freemo Ohh that makes more sense now. Thanks!