Bit depth is an important concept in digital imaging if you want the best possible image quality and if you intend to manipulate images heavily.
24-bit/48-bit image
This is the 'old' way of describing the bit depth of an image, by multiplying the bit depth of each channel by the number of channels. So an 8-bit image would equate to '24-bit' (8 x 3) in the older terminology, and '48-bit' (16 x 3) would be described as 16-bit today.