The gamut or 'accessible colorimetric space' corresponds to the 2D or 3D plot of an ICC profile. Specific softwares make it possible to compare all the profiles of the peripheral along a graphic chain, in order to spot colors or shades which cannot be reproduced on the outlet profile - eg the paper profile.
In this case, an impossible shade to print will be described as 'out of gamut'. The broader the gamut of an ICC profile, the broader the spectrum of the accessible shades which can be reproduced by the peripheral.