What does the initials U2 stand for, as in the band?

"U2" is based on their philosophy. They believe that the audience is part of their music and the concert and that "you too" (U2) are participating in the music.

That info comes straight from Wikipedia...which is just a general opinion. The most popular belief is that it comes from the American or German spy plane. I haven't seen any confirmation as to the exact origin. My understanding is that they (or at least Bono) are rather indifferent, and don't really care for the name.