What is the meaning of subscription in YouTube?

When you subscribe to someone, it just tells you. Most YouTubers use their # of subscribers of saying, "Hey, I have a lot of subscribers, I must have great videos!" or "Hey, I don't have any subscribers, so my videos must suck!" Many people just subscribe out of respect.

You don't subscribe to videos, but to users' channels. Every time any of your subscriptions uploads a new video, you will be told.