A subscription is an agreement conveyed with a signature. It's also buying a bunch of issues of a periodical, such as a year's subscription to "The New York Times" newspaper.
See the word script in subscription? That's a clue that this word has to do with writing. Specifically, a subscription is the act of signing something that you agree to, like a petition. You can have a subscription to a belief too. Also, when you order a magazine or newspaper's future issues, you buy a subscription. Subscriptions like this usually last one year: you pay, and the magazine agrees to send you copies until your subscription runs out.