The most important organic molecules are organic dyes which have conjugated double or triple bonds i.e. double or triple bonds separated only by one single bond. The benzene molecule is the best example of such molecules. Now, a single C-C bond is ~1.54 A in length while a double bond is ~1.33A long. In benzene all C-C distances are equal and 1.39 A.  In traditional chemistry language, we say that the benzene molecule resonates between two equivalent   structures so that all bonds are equal. If we write the wavefunction of carbon atoms, the geometry of the structure suggests that 3 hybridized atomic orbitals are

These orbitals are symmetric about the z=0 plane in which all the nuclei lie. The fourth orbital normal to these is the π orbital

These π- electrons overlap with other π- electrons on either side and are thus free to move on above and below the plane of the molecule as illustrated in the figure 18.1 below. The π- electrons contribute significantly to the chemical and optical properties of these molecules.