There are several levels of masking, with a range of severity. First, a description of keywording.
Keywords are a way of marking a package for a particular architectures. Each architecture has a keyword named after it (x86 for x86-class CPUs, amd64 for x86_64 class CPUs, mips, sparc, and so on). Each version of a package has a list of the keywords that apply to that package. There are four levels of keywording: