The zeta potential of a particle is the overall charge that the particle acquires in a particular medium. The magnitude of the measured zeta potential can be used to predict the long-term stability of the product.
If the particles in a suspension have a large negative or positive zeta potential then they will tend to repel each other and resist the formation of aggregates. However, if the particles have low zeta potential values, i.e. close to zero, then there will be nothing to prevent the particles approaching each other and aggregating.