Stability of resonance structure
Re-read the statement carefully. It says that the separation of opposite charges ("charge separation") destabilizes the resonating structures. The second phrase ("charge delocalisation") refers to the delocalisation of the same charge around a resonating structure.

For example, here's an example of how "charge delocalisation" increases stability:



(source)
Notice the same positive charge is delocalised throughout the conjugated system. It reduces - the unit positive charge on a single carbon atom - to a partial positive charge over several carbon atoms in the resonance hybrid, thus increasing the stability of the molecule.

Here's an example of "charge separation":



which destabilises the molecule. Remember that uncharged structures are more stable than charged structures.

I hope it helps!