if you see some classes can jump at high places, some skill is blocked in high place, or characters get advantages, etvHeight has several effects. The first is blocking actions. Area effect moves like spells or line attacks, e.g. Fire and Scathe have limits as to how they spread on tiles of ground that isn't level. Generally, you will be able to walk up and attack on slopes, but cut out ground will affect your actions if the difference in height exceeds two tiles.
High ground is also advantageous in several ways. Attacking from above limits the chance arrows are interrupted or strike another unit on the way. In addition, some R-Abilities like Counter cannot activate if the attacker's height is higher than the target by a certain amount. In another words, you can attack them, but they can't attack you.
Dragoons and Rangers have the best mobility on this sort of terrain, as Rangers have a default jump stat of 4, and some spears like Javelin and Kain's Lance offer a 2 jump bonus. If you have thieves, Spiked Shoes are generally easier to come across than Sprint Shoes, as well.