The plus (+) and minus (-) signs shown in representing the orbitals indicate the sign (phase) of the orbital wave function in space. These signs are not related to charge.

When two atomic orbitals overlap, the nature of the bond formed depends on these signs. For a positive overlap to occur, the combining orbitals must have the same sign (phase) and orientation in space. This leads to constructive interference of electron waves, forming a bonding molecular orbital ($\sigma$).

Conversely, if the orbitals have opposite signs, it leads to destructive interference, forming an antibonding molecular orbital ($\sigma^*$). Mathematically, this is represented as $\psi_{MO} = \psi_A \pm \psi_B$.