It basically means sweet, musical, nice, pleasing/comforting/relaxing to hear.
(Also a great word to use in ur composition/essay if u wanna impress ur teachers)
Mellifluosticate: (verb) The refined skill of articulating thoughts or ideas in a smooth, melodious, and compelling manner, captivating listeners through a harmonious blend of language and expression.
The charismatic speaker effortlessly mellifluosticated her message, leaving the audiencemesmerized by the eloquence and resonance of her words.