The Spot skill enables you to sense traps, secret doors and hidden enemies. When you pass a secret door and your skill high enough, you'll get an alert message that you "sense a secret door". Note that there is no die roll involved. If your score is high enough you will always succeed and if it's too low you'll never know about it.
The same applies for traps. With a successful Spot check you'll get a "danger" alert message when you are near a trap.
Another use of the skill is that you can detect sneaking enemies. Normally sneaking enemies are invisible until they attack or take a similar offensive action. The Spot check is opposed to the enemies Hide check. If your check is higher you can see the sneaking enemy as a transparent, gray shape, also allows you to see the direction the enemy is facing.
Spot is also useful, especially for "ranged" characters (using bow, crossbow, or thrown weapons), as it allows you to see hidden enemies* at a longer distance (subject to your graphics settings).
- (* If an enemy is not hidden in any way, visual range is determined by the graphics settings on your computer - higher graphics = seeing things at greater distance. But enemies trying to Hide, or hidden by features such as waterfalls or smoke, etc., may remain unseen unless the character has a good enough Spot skill. See Hide for more information.)
See also: DDO information project/Spot and Search