From DDO wiki
When you make a successful Reflex save to avoid damage, you suffer no damage instead of half damage.
- Evasion can be used only if the character is wearing light armor or no armor and is not heavily encumbered. As of Update 23 Evasion will no longer function for characters who have Heavy and Tower Shields equipped.
- A helpless character does not gain the benefit of evasion.
- Evasion primarily helps against certain magic attacks as well as avoiding most types of trap damage. (The spell wards in The Underdark adventures must be met by your Will save and cannot be evaded.) For avoiding most physical and ranged attacks, Dodge bonus chances are used.
- Evasion can also be gained in the following ways: