Imagine HCl molecules floating around in water. H-Cl is a very polar molecule, and it dissociates easily in water because it is polar, and the ions become hydrated, making this dissociation energetically favourable.
One the otherhand, HCl molecules in non-polar methylbenzene, also known as toluene, will not dissociate because there are no charges in toluene to surround the dissociated ions of HCl to make this energetically favourable.
The properties as charco described above.