I first heard this term when I was a teenager and was listening to Frank Sinatra and other popular singers of those times.  There is not a lot to say about it except that it is also “easy singing.”  The voice is never strained, the pitch range is fairly narrow and it is mostly about delivering words and giving an emotional  experience to your listeners. As a teacher, my job with these singers is to encourage them to use their voices with an unrestricted tone quality, teach them to, as we say in singing (see above) color the tone in line with that emotion. I train easy listening singers to develop the various tone colors of their voices with a little work on the strong and sweet tones to assist that vocal color development. It is a most enjoyable exercise for me.