1)Year 2000. (Y=year, 2k=2 times k, k being short for kilo which means 1000)
2)Computers only counted the last two digits of a number, so people thought they didn't know the difference between 2000 or 1900. They thought This would cause various problems with dates and finances, and would cause a huge mess. People were panicking and saying that when 2000 rolled around, all the computers would crash and the nuclear bombs would go off and end the world.
3. It never happened. Dont worry.
December 31 11:58pm :
YOU GUYS WE HAVE TO GET DOWN TO THE CELLAR!
Y2K IS ABOUT TO HAPPEN!
January 1 1:03am :
It didn't happen.