Colony Collapse Disorder is when most the worker bees from a beehive go away and never come back - mysteriously. The hive dies. The bees are never to be found.
The term "Colony Collapse Disorder" (or CCD) was first applied to a horrible rise in the number of disappearances of honey bees in North America in 2006. It has spread around the whole world. The loss of honey bees is important because most our food is pollinated by bees, and our meat comes from animals that eat food pollinated by bees.
There are places in China where they no longer have honey bees from pesticide use and the farm economy in those areas have completely collapsed, except where they pollinate some pear trees slowly by hand.
CCD in the rest of the world is harder to pin down to one cause and they think it might be a combination of things.
1) It might be other insect diseases - some calling it an insect version of AIDS. Today's honey bees often have a mold growing in them that is common in humans with AIDS.
2) There's environmental change related stresses that might lower their immune systems, too.
3) There's malnutrition from just living on one type of a crop's pollen, from factory farming.
4) Pesticides cause huge death tolls to bees and other pollinating insects.
5) Cell phone radiation is being considered. It might make the bees get lost.
6) Genetically modified crops with pest control characteristics might be as deadly to bees as direct pesticides.
7) There might be other chemicals in the environment that give them learning disorders so they fly away to find pollen but they forget what they're doing and can't get back home.