A guttural language is a language that contains guttural sounds-- these are produced from the pharynx at the back of the throat or by the back of the tongue, as well as the palate and uvula. The word guttural is derived from the French and Latin denoting coming from the throat. Sometimes whether a language is guttural or not could depend on differences within regions and countries. The study of "gutturals" falls under the field of phonetics.
Thus, it means that a pronounced or heavy sound, letter, and word that emanates from the pharynx and up from the back of the throat makes a certain kind of very heavy and pronounced "throaty" sound, very alien to the modern spoken English language and sometimes found to be very hard on the ear of those accustomed to English sounds and pronounciations.
The concept of gutturality is not entirely objective, but a guttural sound is generally believed to be one which is pronounced with the dorsum of the tongue and/or at any point behind the hard palate, including the soft palate, the uvula or the pharynx.
English too has its gutturals, such as the letters "k", "c" (when it sounds like "k"), "q", "g" as in "go", and the "ng" and "nk" at the end of a word, and the vowels "a", "e", "i" ,"o", but they are all considered to be very "soft" sounding gutturals, unlike those of other languages that stress them.
In the popular consciousness, some languages are considered to be guttural, as opposed to just possessing some sounds which are pronounced at the back of the oral cavity. Often, this is just a result of the beliefs of Anglophones or of non-speakers of those languages. Some languages which have fallen under the popular meaning of "guttural", as opposed to the technical meaning, are German, Klingon, Ubykh and Arabic.
French, Arabic, Hebrew, Scots, and also partly German, Dutch, Afrikaans, and Yiddish, all contain sounds that come from the back of the throat or tongue. Sometimes whether a language is considered guttural or not could depend on differences within regions and countries. In French, the only truly guttural sound is a voiced uvular trill; Arabic and Hebrew both contain rather more gutturals, including velar, uvular and pharyngeal fricatives.
See also
Articulatory phonetics, linguistics, place of articulation, consonant, fricative consonant, glottis, trachea, esophagus, phone
