A subscriber is fan or friend of a certain YouTuber. Sometimes, people subscribe just to see the latest content for whatever reason. No matter for what reason, subscribers increase popularity of a channel. PewDiePie currently has the most subscribers on YouTube, excluding some of YouTube's hashtag channels (#Gaming, #Music, Movies).
Top Subscriber Count
Top YouTubers Subscriber Count
Usually, subscriptions do not last forever. It is always a likely chance that someone will get bored of your channel. When this happens, they will either rarely watch your videos, unsubscribe, or be a ghost follower.
Sometimes, subscribers unsubscribe if the channel becomes somewhat inactive.
Pretty much, this only effects small channels. Big channels, who have over 1,000 may not even notice. And usually, when they do notice, they don't care; they have a lot more left. YouTuber McJuggerNuggets, for example was reacting to mean comments. When someone said that he unsubscribed, McJuggerNuggets clearly said that he did not care. McJuggerNuggets is one of the most hated popular channels on YouTube, because the videos he posts is his daily life. He would not listen to his dad, because his dad always bashes his things.
Now, it depends on who agrees with who.
People who agree on his dad hate him, because he needs a life and job.
People who agree with McJuggerNuggets like him or feel sorry for him, so they subscribe.
However, McJuggerNuggets was not careful of what he is filming in his videos. The video, "KICKED OUT OF LARRY'S" changed his fandom quite a bit. That's when people started to leave his channel.
Markiplier, on the other hand, is different to most YouTubers. He cares a lot about his fans. He is sometimes sad to figure out he loses subscribers. But, usually he does not notice. In his 8M Subscriber Special, he said while crying, being honest of how he feels, that he wished he knew all of his subscribers personally. But he can't. And that is what makes him sad. He repeated it a few times in that video.
A Guide into Getting Subscribers
Have good commentary: That is always one of the basic steps of getting your fans.
Reply to your comments: Replying to comments will keep people on your channel. It'll make them know you care a lot about his/her fans.
Be Careful: Be aware of what you are doing in your video. Make sure you are not acting stupid, or somehow insulting your subscribers, or being rude (Like the McJuggerNuggets example above).
Showing your face: You can still get subscribers if you don't show your face, but you'll grow faster if you do. It will make your subscribers feel like they are in a deeper connection with you.
Make Subscriber Specials: It is always polite to thank your fans for subscribing to you, it will show that you care about them, and they are not just strangers watching your videos.