When some server on the Internet gives your browser a file, it includes a metadata called an expiry header (actually, Expires:), which the browser is supposed to interpret as providing guidelines for caching. For example, if I tell you that this web page is good for until midnight tomorrow, and the user requests the same page later today, the browser can (but doesn't have to) serve the user the page out of the local cache, secure in the knowledge that I have told it I would not update the page until at least midnight tomorrow. Since this totally skips the HTTP request for the page the second time, it results in blazing bugs-in-your-teeth speed.
Saying JQuery's expiry time is 1 year in the future means that Google is telling browsers who get the JQuery script "Hey guys, you can rely on this being constant for a year, after that you'll probably want to ask for a new version." The poster above you apparently thinks this is suboptimal, because a user should be able to cache a particular version of JQuery for longer than one year. I think that it is irrelevant, since in actual practice almost no user agent will reliably persist a cache for a year, and 1 extra HTTP request a year isn't enough to worry about, anyway.
