It is the angle of the main wing or wings to the root or center line.

if you had a flat wing meaning laying flat on a table it would have 0 degrees of DIHEDRAL.. if you lifted each wing tip say 3/4 inch that is with the center of the wing still touching the table, you would have a dihedral of 1.5 inches. the degrees would depend on the length of the wings. I am sure there will be much more technical answers but am trying to answer this simply. It is the factor that helps some planes as they say Self correct. Planes that use just rudder for turning have a great deal of it.

the DIBO

I THINK ) :spinnyeye