Around two years ago YouTube introduced new option named YouTube RED, which in fact is kind of premium subscription, which gives you some advantages which aren't available for standard YouTube users such as playing videos in background on mobile phone or downloading videos for offline watching. In the nearest future YouTube probably will introduce new option named YouTube Plus, but in fact we still don't know what exactly it will be.
Some people noticed in code of YouTube apk file in 12.37 version first lines about this potential new option. This option according to this lines will allow to play videos in background, so we can guess that YouTube Plus may be cheaper version of YouTube RED subscription. Probably YouTube will express its position on this rumors, so we have to wait for this moment to receive more confirmed information.