Gamut:

Gamut is the total range of colors produced by a device.

For any printing device there will be physical limits to the range of color that can be reproduced. In the above illustration, what we see is the difference between the bright light of a monitor shooting bright electrons at us vs. the duller effect of ink on paper.

RGB Gamut is larger than CMYK Gamut

If your CD or Booklet design files contain RGB colors when we receive them, 
they must be converted to CMYK at an additional cost.

A color is said to be "out of gamut" when its position in one device's color space cannot be directly translated into another device's color space. For example, the total range of colors that can be reproduced on a monitor or color RGB laser printer is greater than that for an offset press. A typical CMYK gamut is generally smaller than a typical RGB gamut. This is the primary reason that colors go dark and dull when you convert an RGB image to CMYK. Most notably, certain vibrant deep blues and rich reds are "outside the gamut" of SWOP CMYK.

SWOP stands for Standard Web Offset Press.