If the wave functions reinforce each other, the atomic orbitals (AO) are in phase and their combination results in the formation of a bonding molecular orbital (MO). A bonding MO is lower in energy than the AOs that combined to form it. The electrons in this type of MO contribute to the strength of the bond
If the wave functions cancel each other, the AOs are out of phase and their combination results in the formation of an antibonding MO (with a node). An antibonding MO is higher in energy than the AOs that combined to form it. The electrons in this type of MO impair the strength of the bond