Patina is the oxidized metal that coats many ancient bronze and copper coins. It is usually green, but can be many colors including blue and red. The main reason you want to preserve it is that it is usually attractive and the coin under the patina usually isn't as nice. A coin with an attractive patina is often worth more than a coin that does not have a patina at all. A common coin, like a common Constantine dynasty AE3, that has been stripped of its patina down to bare shinny metal is almost worthless. See the extended definition below.
http://www.forumancientcoins.com/numiswiki/view.asp?key=patinaCopied