I think they are asking for 8 bit per channel image. Three channels multiplied by 8 bit (Red, Green, Blue) gives you number 24. Read about color depth from here https://en.wikipedia.org/wiki/Color_depth. Sometimes people are reffering to PNG as 24 and 32 where 8 additiona bits are for transparency (R, G, B - 8 bit channels plus transparency).
"RGB images with 8‑bits per channel (Bits/Channel or bpc) are sometimes called 24‑bit images (8 bits x 3 channels = 24 bits of data for each pixel). "