Rage is a percent bar that fills up as you take damage from enemies, and when it reaches 100% it triggers a buff called Burning Wrath, which temporarily boosts your attack/defense stats and adds an orange glow around your character. Rage builds are common for players with high defense.
The duration of Rage is usually very short, however it can be increased by the Burning Wrath ability that many classes have.
During Burning Wrath, your attack, defense, and skill defense stats are all increased by 20%. Rage does not increase dodge rates, nor any other stats. If you are a Spearman, the Reigns of the Violent Gale ability will add additional stat increases during Burning Wrath.