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 they didn't know the difference between 2000 or 1900. This would cause various problems with dates and finances, and would cause a big mess.
People were panicking and saying that when 2000 rolled around, all the computers would
crash and the nuclear bombs would go off or something.