The phrase "out of gamut" refers to a range of colors that cannot be reproduced within the CMYK
color space used for commercial printing. Graphics software is designed to work with images in the RGB
color space throughout the editing process. The RGB color space has a much wider range of discernible colors than CMYK. When you print an image it must be reproduced with inks and these inks cannot reproduce the same range of colors that we can see with our eyes. Because the gamut of color that can be reproduced with ink is much smaller than what we can see, any color that cannot be reproduced with ink is referred to as "out of gamut." In graphics software, you often will see an out of gamut warning when you select colors that will shift when an image is converted from the RGB color space used in the editing process, to the CMYK space used for commercial printing.