The roots of verbs and most nouns in the Semitic languages are characterized as a sequence of consonants or "radicals" (hence the term consonantal root). Such abstract consonantal roots are used in the formation of actual words by adding the vowels and non-root consonants (or "transfixes") which go with a particular morphological category around the root consonants, in an appropriate way, generally following specific patterns. It is a peculiarity of Semitic linguistics that a large majority of these consonantal roots are triliterals (although there are a number of quadriliterals, and in some languages also biliterals).
Such roots are also common in other Afroasiatic languages. Notably, while Berber mostly has triconsonantal roots, Egyptian and its modern descendent, Coptic, both prefer biradical and monoradical roots.