What’s Mellow?

During a survey, which of my guitars would have the most interesting sound, I got several interesting answers. “Lucid Gypsy” (let me call her “Mellow Gilly”) for example answered: “It’s hard to choose but the one with the light wood and f holes because its so mellow.”

P.S.:
English is a foreign language for me, so sometimes I have to analyse a word more than others –
MELLOW:
1
make or become mellow = “getting older does mellow the hard edges around the anger”
2
(especially of sound, taste, and color) = pleasantly smooth or soft; free from harshness. “she was hypnotized by the mellow tone of his voice”
3
dulcet, sweet-sounding, tuneful, melodious, mellifluous, soft, smooth, warm, full, rich
4
(of a person’s character) = softened or matured by age or experience. “a more mellow personality”
5
genial, affable, amiable, good-humored, good-natured, amicable, pleasant, relaxed, easygoing, low-maintenance, placid, jovial, jolly, cheerful, happy, merry