The plus and minus sign indicate the sign of the wave function in the various lobes and not that of the nuclear or electronic charges. The ‘+’ sign (or upward crest) represents one phase and the ‘–’ sign represents second phase of the wave function. Bonding molecular orbital is formed by combination of ‘+’ with ‘+’ and with ‘–’ part of the electron waves whereas antibonding molecular orbital is formed by combination of ‘+’ with ‘–’ part of the electron waves.
As orbitals are represented by wave functions, a plus sign in an orbital represents a +ve wave function and a minus sign represents a –ve wave function.