What is the mean of positive and negative sign in lobes (I have attached the image).

The two lobes of a p-orbital are different in phases.One phase is represented by a positive sign,while the other is represented by a negative sign.These two phases can as well be represented by different colors.Thus,a positive overlap is an overlap between orbitals which results when orbitals in the same phases overlap with each other.Bond formation occurs due to positive overlap between orbitals.
A negative phase overlap results when two orbitals with different phase overlap with each other.A zero overlap will results when orbitals do not overlap at all or do not overlap effectively.A zero overlap of orbitals does not result in bond formation.