It means the mask is applied to the variable being masked and bacause it is an AND, only the bits that are a 1 in both number appear in the result eg assume the number 26 for d[0]

00011010 = 26 = d[o]
00000111 = 7 = 1st mask

00000010 = 2 = 1st result

00011010 = 26 = d[o]
00001000 = 8 = 2nd mask

00001000 = 2 = 2nd result

Hope that makes sense