Melisma is a Greek word that describes a singing technique in which the singer produces a number of different notes within the same syllable in the lyrics, it was typical of ancient singing, especially in religious contexts to induce a trance to the listener , we can hear it in Gregorian singing for example and typically in Arabic chants. It is not much in use nowadays, but there are some artists who use it up to a point, Stevie Wonder and Mariah Carey are good examples. It is "opposed" to syllabic singing, in which the singer produces only a note for every syllable, most western singing traditions are syllabic.
Melisma has nothing to do with the vocal range or skill of the singer, it is just a different technique coming from a different tradition and in fact critics don't usually praise the overuse of it.