If the mass commercialization of jazz instruction has a decisive moment, it would probably be the arrival in 1967 of a play-along album and guidebook called ''How to Play Jazz and Improvise.'' The recording featured a rhythm section only, leaving room for anyone to fill in the blanks. Though not the first effort of its kind -- a company called Music Minus One was already in business -- it was quickly the most successful, and influential.
(Jazz Is Alive and Well. In the Classroom, Anyway. The NY Times)
What is the meaning of "play-along" in the above?