A gamut is the range of colors:
The gamut, or color gamut is:
In color theory, the gamut of a device or process is that portion of the color space that can be represented, or reproduced.
In the case of a computer display, a gamut is all the colors it can accurately show.
Visualizing a gamut is a bit complicated, but it is slightly similar to the color wheel
A gamut is the area on color wheel surface that represents as far as the device can go in all directions.
You might see a gamut represented with a diagram such as the below one, where:
When certain colors cannot be expressed within a particular color model, those colors are said to be out of gamut. For example, while pure red can be expressed in the RGB color space, it cannot be expressed in the CMYK color space; pure red is out of gamut in the CMYK color space.