Dual plane means that the area on the manifold under the carby (the plenum) is divided into 2 sections (dual). One side feeds 4 cylinders, the other half feeds the other 4. It gets a little spaghetti like under there, ie: when feeding 4 cylinders, there will be 2 on the left and 2 on the right.
On a single plane the plenum area is wide open and feeds all 8 cylinders.
The other version of the single plane manifold is the individual or single runner. See pics below.
Generally the dual plane manifolds promote more torque down low where as the single planes are better suited to higher revving performance motors. They have a straight shot into the heads.....the dual planes become restrictive. ie: Dual plane goor for 1000-5500rpm, Single 2000-7000, Single Runner 3500-8000 these are rough numbers and vary between manifolds.
Dual Plane (Edelbrock Performer)
Single Plane (Weinad Xcelerator)
Individual runner design