In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands.

Such a repulsion is more when the metal d orbital is directed towards the ligand than when it is away from the ligand. Thus, the ${\mathrm{d}}_{{\mathrm{x}}^2-{\mathrm{y}}^2}$ and ${\mathrm{d}}_{{\mathrm{z}}^2}$ orbitals that point towards the axes along the direction of the ligand will experience more repulsion and will be raised in energy; and the ${\mathrm{d}}_{\mathrm{xy}}, {\mathrm{d}}_{\mathrm{yz}} \text{and} {\mathrm{d}}_{\mathrm{xz}}$ orbitals which are directed between the axes will be lowered in energy relative to the average energy in the spherical crystal field.

Thus, the given diagram explains d orbital splitting in an octahedral crystal field.