Basting spray is an aerosol glue you can use to spray the layers of your quilt so they stick together. It's a faster basting method in comparison to pin basting or hand basting, and I know many quilters that prefer to use spray basting because it's easier on their hands.
As with all my Great Quilting Debate episodes, I 100% believe you should use what you want and what works for you! I use polyester thread for quilting, I like to prewash and starch all my fabric before cutting, and I press my seams open - all of these are topics that quilters argue about.
But I rarely hear anyone arguing about basting spray and this worries me. I get questions weekly about whether starch will make their quilts more susceptible to bugs (a ridiculous quilting rumor in my opinion), but I never get questions about the effects of basting spray - a far more detrimental chemical.
Maybe this is because I don't use basting spray so quilters are taking their questions about it elsewhere. Or maybe quilters don't know about the possible side effects of using this chemical in their quilts. Read more about spray basting here: http://freemotionquilting.blogspot.com/2018/01/is-basting-spray-bad-for-quilts-podcast.html