What is meant by "charge imbalance?"
It means exactly what the words say: the charges do not all add up to zero. Ions have such a charge imbalance, as they have definite positive or negative electrical charge.
An everyday experience with charge imbalance is static electricity generated from rubbing one's hair with a balloon.
Back in the day, I had a special pillow that I could rub against my hair, and it would spark in the dark.