If we have two bodies with charge q1 and q2 on them and we are given q1 + q2 = 0. It means the net charge on the system is zero.
This is possible only if q1 and q2 are equal and opposite.
That is, q1 = +q and q2 = -q or vice versa.
Such that, q1 + q2 = +q + (-q) = 0