Color depth (or bit depth) is the number of bits per channel (bpc) used to represent the color of a pixel. The more bits for each RGB channel (red, green, and blue), the more colors each pixel can represent.
In After Effects, you can work in 8-bpc, 16-bpc, or 32-bpc color.
In addition to color bit depth, a separate characteristic of the numbers used to represent pixel values is whether the numbers are integers or floating-point numbers. Floating-point numbers can represent a much larger range of numbers with the same number of bits. In After Effects, 32-bpc pixel values are floating-point values.
8-bpc pixels can have values for each color channel from 0 (black) to 255 (pure, saturated color). 16-bpc pixels can have values for each color channel from 0 (black) to 32,768 (pure, saturated color). If all three color channels have the maximum, pure-color value, the result is white. 32-bpc pixels can have values under 0.0 and values over 1.0 (pure, saturated color), so 32-bpc color in After Effects is also high dynamic range (HDR) color. HDR values can be much brighter than white.