The Material Plane was what we know as the physical plane, or where most of our living experiences take place. This is only of particular note with regards to mythology and magic use. The material lay is thought to be the main plane, while the others are simply Overlay Realms.
Magic users could extend themselves beyond this plane into places such as the Ethereal Lay. Some creatures that lived in other planes, such as wraighs, could be summoned to the material plane.