Following is a standard Mastodon feature, you follow as usual just like you'd do on Twitter. The followed person gets notified. Or if it needs approval, needs to approve before you can actually follow.
Subscribe doesn't notify the person, doesn't need approval, and you can only see their public toots. You won't see their follower-only posts. It's a Qoto-specific feature.
Super thank you @trinsec !!
Nice feature!!