Specifically I'm trying to get a better understanding of Exchange Web Services (EWS) and I see many references to EWS subscriptions. It appears that a 'subscription' is a generic HTTP concept but I haven't had much success searching 'subscriptions' online.
Without the context (link to a document you're seeing this term?), I have two educated guesses:
You may be referring to RSS feed subscriptions, which are (in one use) a way of maintaining a easily-parsed listing of updates on a website. I believe that you can also turn a mailbox into an RSS feed with EWS.
You may also be referring to push or pull subscriptions within the EWS API, meant to allow developers to query for change events within the mailbox store. Unless you're coding an application that will interface with EWS, this is likely not what you're looking for.