Stamp collecting comes in so many different forms, sometimes it can become quite bewildering. Overprints can be among the trickiest aspects of stamp collecting. The Malaysian online newspaper The Star, has a nice article specifically about overprints in today’s edition. You can read it here: Special mark of overprint. The article is very well done, even if somewhat hard to read due to the English.

From AskPhil.org
Overprint (Opt.,Ovpt.): an additional printing on a stamp that was not part of the original design; for example, the Molly Pitcher U.S. stamp of 1928 has the words “Molly Pitcher” overprinted on top of the design; where the additional inscription changes the face value of the stamps, this is then named a surcharge.