Chill out music emerged in the early and mid-1990s in "chill rooms" at dance clubs, where relaxing music was played to allow dancers a chance to "chill out" from the more emphatic and fast-tempo music played on the main dance floor.
The genres associated with chill-out are mostly ambient, trip-hop, nu jazz, ambient house, New Age and other sub-genres of downtempo. Sometimes the easy listening sub-genre lounge is considered to belong to the chill-out collection as well.
Chill out as a musical genre or description is synonymous with the more recently popularized terms "smooth electronica" and "soft techno" and is a loose genre of music blurring into several other very distinct styles of electronic and lo-fi music.
Source: Wikipedia, YouTube