Contract to receive and pay for a given number of issues of a periodical, for tickets to a series of performances, or for a utility service. For example, We subscribe to the local paper, or Betty and I have been subscribing to this concert series for years, or We have no choice; we have to subscribe to the local power company.
Feel or express approval of, as in I subscribe to your opinion but I don't think Donald does.
Promise to pay or contribute money to, as in We subscribe to many charities.
All of these usages come from subscribe in the sense of “sign one's name to something, such as a pledge.”