An administrator is a special kind of editor, with power not only to edit a page, but block it from other editing, and block other editors who vandalized pages - for example, writing obscenity on a page.
Why is blocking a page such a drastic step? The answer is because no one can add information to or, again, correct an error on a blocked page. There are "talk" pages on Wikipedia, on which someone can call attention to an error that needs correction - but only an administrator can make this correction.
Why is this so important? Santorum is, by any standard, one of the two leading candidates for the Republican Presidential nomination. His Wikipedia page has been read more than a million times in the past 30 days. That's pretty important. If there's an error on his locked page, or new information that needs to be added, tens of thousands of people a day will be getting this wrong or incomplete information.
How would one know that this page is locked? There's a little gold lock in the upper right-hand corner.
Why was it locked? My wife, who edits on Wikipedia, has been trying to find out. (She was one who told me this page was locked, a few hours ago.) You do this by asking on the talk page of the blocked article why it was locked. Or on an administrator page. So far, there has not been much of an answer.
Wikipedia, as I wrote in my 2009 book New New Media - I'm currently finishing a new edition - revolutionized the way we get knowledge by operating on reader/writer consensus rather than editorial fiat. When administrators fully block a page, this cuts at the very heart of Wikipedia's advantage - not to mention our democratic process, which depends on accurate information being available to the voting public.
The page may be unlocked by the time you're reading this - but it's locked right now, and this whole affair requires some looking into.
Note added at 3:30pm, February 20: Rick Santorum's page was just changed from "Full Protection" to "Semi-Protection," after objections to the full protection raised by several editors including my wife. The reasons for the full protection are not entirely clear, but "edit wars" (repeated deletion and reinsertion of the same text) over Santorum's positions on contraception were at least a contributory factor. "Semi-protection" a much less extreme form of protection, in which any one can edit, as long as they have an account (Wikipedia allows people to edit who do not have accounts), which is at least 4 days old and has made at least 10 edits on Wikipedia. The change to semi-protection is a big improvement, but still interferes with Wikipedia's ideal of anyone being able to update a page, or correct an error.