Covalent bonds are 'electron sharing' bonds, as the name implies, but the sharing doesn't have to be equal. Any tendency for the shared electrons to favor one side of the bond over the other in their average distribution will lend an asymmetry to the arrangement of the atoms (if there are three or more atoms participating in the bond). A good example of a directional covalent bond is water, where the molecule has an angular structure due to the asymmetric distribution of electrons between hydrogen and oxygen.
or,
Along the bond axis.
(Unlike the charge in an ionic bond which is radial, or omni-directional.)
It comes into play when determining the polarity of a molecule.
If the directionalities of polar bonds cancel out (vector addition to 0) the molecule is non-polar even though the bonds are polar.
