What do the 4 letters on a dreidel mean?

The 4 Hebrew letters on a dreidel stand for, “Nes Gadol Haya Sham,” meaning “a great miracle occurred there.” Dreidels are used for a traditional Hanukkah game.

Saturday, February 04 2012
Source: http://www.myjewishlearning.com/holidays/Jewish_Holidays/Hanukkah/At_Home/Dreidel/How_To_Play.shtml