In battings, it is a thin piece of fabric through which the batting is needle-punched. Needle punching batting makes it more stable. Needle punching batting through scrim makes it even more stable.
Warm n Natural is an example of a batting that is needle-punched through scrim. It is very, very stable so needs to be quilted only 10" or so apart, or a quilt with this kind of batting can be tied. Scrim can give a batting a slightly more stiff drape.
My favorite batting is Mountain Mist Blue Ribbon 100% cotton, a very old-fashioned batting that has been in use for many decades. It is not needle punched and therefore needs to be quilted about every 2". It would not be a good batting to use in a tied quilt, because the batting would have a tendency to ball up with washings and use. It needs close quilting to keep it in place. I love it, though, because it becomes softer with every washing. After about 10 years of hard use, a quilt with this batting is soft as a cloud!