Use Magic Device, or UMD, is a character skill in DDO that allows you to use a wand, scroll or restricted item even though you would not normally be able to.
What that means is that with a high enough UMD you can cast scrolls of Heal or wands of Cure Light Wounds even though you are a rogue or sorcerer, you can equip Elvish race-restricted weapons as a Warforged or Chaotic alignment-restricted items as a Lawful character. With a high enough UMD you can essentially expand your character’s utility immensely.
If you are serious about accessing as many magical devices as possible especially as a non-magic-using class you will want to shoot for a buffed UMD score of 44, which is currently the highest ranked item in the game – the Scroll of Resurrection. However having a total UMD rank of 40 will still allow you to use basically everything else.
If you do not plan to use high-level scrolls or restricted items or if your overall character design will not feasibly allow it, a better method is to target the UMD Difficulty Check (DC) of the specific items you plan on using. It may turn out you really only need a total rank of 20.
To determine the UMD DC of an item, simply double its level and add 18. Or rather -
(item_level * 2) + 18 = DC
To successfully use this wand as, for example, a Rogue you would need to roll higher than 20 on a d20+UMD. If you are not familiar with dice shorthand that means you roll one 20-sided die and add your total UMD rank. What that also means, therefore, is that if you have a total UMD rank of 20 you will be able to use this wand 100% of the time without failure.
What makes up a character’s “total” UMD skill rank?
UMD is determined by adding the ranks you spend points on after leveling, your Charisma modifier, the Skill Focus: UMD feat and a very few rare items in game which have a UMD skill boost.
It is a primary skill of Rogues and Bards and a secondary skill for everyone else, meaning they will only spend one point to raise their UMD a rank while all other classes must spend two skill points.
Rogues and Rangers can temporarily boost UMD through their respective Action: Skill Boost enhancements. Humans get the same type of boost through the Human Versatility Bonus enhancement and Bards can get a boost to UMD via the Spellsinger enhancement (at level six). But other than that, UMD bonuses are pretty hard to come by, so if you are dedicated to having a high UMD, make sure to always reserve your skill points to keep it maxed for your level and consider assigning Skill Focus: UMD as one of your feats.
So if you decide to emphasize UMD in your DDO character build, keep this in mind. Sometimes it could be worth having the one-level splash of Rogue or Bard for this among their other talents.
Also keep in mind when designing your character, that if you play a class with casting abilities you already have access to wands and scrolls of spells your class has access to. As an example, even though as a Ranger you start at level 1 with no spells you will be able to use the Wand of Cure Light Wounds because it is one of your class spells you will later be able to cast.