overlapping and superimposing are kinda the same thing, except the latter emphasises more on wave mechanics.
in a region where two orbitals of both + signs overlap, electron density is higher than the mere sum of the electron density of the two seperate orbitals. More electrons density is shared between two atoms. the attraction of both nuclei for these electrons is greater than the mutual repulsion of the nuclei, and a net attractive fore aka bonding intereaction occurs.
in regions where two orbitals of both - signs overlap, there is effectively no electrons between the nuclei, Hence, there is only repulsion force between the 2 nuclei, hence no bonding occurs.
in regions where two orbitals of opposite signs overlap, the electron density present between two nuclei might not be sufficient to produce an attractive force stronger than the repulsion force between the nuclei. The electron density between the two nuclei in this molecular orbital is less than the sum of individual electron density of each orbitals. Hence, no bonding occurs.