What is meant in Spanish phonology by the debuccalization of /s/ to [h]? What dialects does this phenomenon primarily occur in? In those dialects, does it take place in all cases or only in some environments?
Debuccalization is a sound change that consists in a consonant losing its original place of articulation becoming [h] o [ʔ].
Saying place of articulation, we mean one of these:
In most places of South America (except Mexico, guatemalan plateau, Costa Rica, andean Venezuela, Quito and ecuadorian plateau1, bolivian plateau and Bogotà) and in half of the southern Spain, the s in the last position of a syllable is an aspirated (a voiceless glottal fricative, /h/, that is, debuccalization) or sometimes not pronounced at all in some variants of the current language. For example, see the following sentence:
In the eastern Andalusia and in the Murcia region, the distinction between syllables with silent s and those originally without s has been preserved in the pronunciation of syllables ending in s with open vowels (that is, the contrast between open and closed syllable turned into a tense/lax vowel contrast. This typically affects the vowels /a/, /e/ and /o/, but in some areas even /i/ and /u/ have, in effect, a double set of phonemes.