It means what the cross section of the blades looks like. Just like wings, blades have a airfoil shape to them.

Symmetrical ones are the same on the top half as on the bottom.
Symmetrical blades work just as good inverted as they do rightside up.

Semi symmetrical ones have different shapes top and bottom.
Semi symmetrical blades create more lift rightside up than they do upside down.

Both kinds of blades probably can work inverted, depending on your negative pitch, but symmetrical blades will perform just as good inverted as rightside up.