A quilt “sandwich” occurs when you layer the backing, batting and quilt top. After these three sections are layered, the quilt is ready for quilting either by hand or machine. The quilting is needed to prevent the layers from shifting during washing.
This is a photo of a quilt sandwich; the backing, batting and quilt top are pinned before machine quilting.

click image to enlarge
Also see:
- Beginner Basics – Quilt Sandwich Tutorial