Range generally refers to the distance a skill requires to have an effect, or the distance between a player character and their target.
Attacks that don't require direct contact are called ranged attacks, as opposed to melee attacks.
In Aion, range is measured by the metric system.
You can see a character's range from you in their tooltip by hovering the mouse cursor over them. This will also display their relative altitude to the player.
Additionally, to display a target's range next to their their health bar when targeting, select: