From DDO Compendium

An outfit is the typical garment appearance for Monks. Most outfits appear as martial-arts wear. Outfits are similar to robes, which often requires only a level proficiency to use them. Outfits typically are built with melee defenses such as Energy Resistances, damage guards (an effect triggered when hit), or augmentations to offensive damage or saving throws. In comparison, robes often grant attributes that support their spell casting abilities. Unlike armor, outfits (and robes) can be donned instantly, with no wait time to equip them.