In crystallography, interstitial sites, holes or voids are the empty space that exists between the packing of atoms (spheres) in the crystal structure. 
The holes are easy to see if you try to pack circles together; no matter how close you get them or how you arrange them, you will have empty space in between. The same is true in a unit cell; no matter how the atoms are arranged, there will be interstitial sites present between the atoms. These sites or holes can be filled with other atoms (interstitial defect).  The picture with packed circles is only a 2D representation.  In a crystal lattice, the atoms (spheres) would be packed in a 3D arrangement.  This results in different shaped interstitial sites depending on the arrangement of the atoms in the lattice.