How do you explain the CMB anisotropy diagram?

[Simfish]

http://map.gsfc.nasa.gov/media/070950/070950b.jpg
So the x-axis is multipole moment, which I remember from 2nd-quarter electromagnetism. Anyways, it surprises me that 1000-order multipole moments can even be physically significant.

 

[Simfish]

I got a professor to explain most of it to me. But I still don't get "opening angle". You supposedly can get a value with an "opening angle of 20". Or 21. Or 22.