A phrase indicating affirmation, enthusiasm or just general agreement.
Probably first used in the Midwest USA (Kansas City, St. Louis, Minneapolis etc.) in the early 70s where it was spoken by radio station DJs and was initially popular with high school and junior high school girls. It was spread primarily through TV and radio given that it was coined before the world wide web was in use. Now, occasionally it is heard world wide in the USA and the Anglophone Common Wealth. Although it is not a common phrase, it is usually understood, evidently, since it employs the widely recognised adjectival 'cool'.