Canon EOS 90D Advanced User's Manual page 390

Customizing Playback Information Display
 [Brightness] display
This histogram is a graph showing the distribution
of the image's brightness level. The horizontal
axis indicates the brightness level (darker on the
left and brighter on the right) while the vertical
axis indicates how many pixels exist for each
brightness level. The more pixels there are
toward the left, the darker the image. The more
pixels there are toward the right, the brighter the
image. If there are too many pixels on the left, the
shadow detail will be lost. If there are too many
pixels on the right, the highlight detail will be lost.
The gradation in-between will be reproduced. By
checking the image and its brightness histogram,
you can see the exposure level inclination and
the overall gradation.
[RGB] display

This histogram is a graph showing the distribution of each primary
color's brightness level in the image (RGB or red, green, and blue). The
horizontal axis indicates the color's brightness level (darker on the left
and brighter on the right), while the vertical axis indicates how many
pixels exist for each color brightness level. The more pixels there are
toward the left, the darker and less prominent the color. The more pixels
there are toward the right, the brighter and denser the color. If there
are too many pixels on the left, the respective color information will be
lacking. If there are too many pixels on the right, the color will be too
saturated with no gradation. By checking the image's RGB histogram,
you can see the color's saturation and gradation condition, as well as
white balance inclination.
390
Sample
Histograms
Dark image
Normal brightness
Bright image