"Chill-out (shortened as chill; also typeset as chillout or chill out) is a loosely defined form of popular music characterized by slow tempos and relaxed moods. The definition of "chill-out music" has evolved throughout the decades, and generally refers to anything that might be identified as a modern type of easy listening. Some of the genres associated with "chill" include downtempo, classical, dance, jazz, hip hop, world, pop, lounge, and ambient."
https://en.wikipedia.org/wiki/Chill-out_music
To me on the otherhand chill music is a bgm to amplify your mood.
Truly, in most cases ambient artists are going for the easy/happy feelings/songs but still. Sometimes your mood is not requiring overly happy bgm to think.