A subscriber is known as fan or follower of a certain channel or content creator. When a user "subscribes" to a channel, it would mean that they grant permission for receiving updates for whenever a YouTube channel uploads, and can access this on their "subscription" feed on the site. Most times, an individual would subscribe to a channel regardless of whether they're a fan or not and would simply want to be updated on what the YouTuber has been uploading. Regardless, subscriptions increase the popularity of a channel.