Color texture format, 8-bits per channel.
Each of RGB color channels is stored as an 8-bit value in [0..1] range. In memory, the channel data is ordered this way: R, G, B bytes one after another.
Note that there are almost no GPUs that support this format natively, so at texture load time it is converted into an RGBA32 format. RGB24 is thus only useful for some game build size savings.
Additional resources: Texture2D, texture assets.