Fabric scale refers to the size of the design on the fabric. Solid colored fabric has no scale because there is no design on it. Calico fabric usually has a small scale pattern. Most decorator prints are larger in scale.
Quilters usually consider the scale of the fabric pattern when choosing material for a quilt. Small fabric scale prints work best in small pieces. Larger pieces will show off your large scale prints.
A video discussing scale can be found at: