can someone explain what this is becuase im making my theme for the psp and don't know what this is.

ill explain what it is but you don't need it and ill explain that too

alpha is what tells a picture veiwer what's visable and what's not... depending on what program your using its done differently
you don't need it at all if you do this..
skip using a bmp in favor of a png and you just make the pic on a transparent backround and the backround when loaded into rco edit will remain transparent even if it looks white on your pc...