A bitmap is literally a map containing bits, just zeroes and ones. How the bits are interpreted (“mapped to” a visual image) depends entirely on the graphic mode used to display the bits.
The bitdepth of a mode refers to how many bits are used to define the information for a single pixel in the bitmap.
The short notation to describe the bitdepth of a digital image/bitmap is bpp (bits per pixel).
In an indexed image, the bitdepth directly defines the maximum number of different colors any pixel can be: It is “two to the power of bpp”, provided the palette contains that many distinct colors.
All three modes in the example here use an identical bitmap.