Certain effects apply a condition to its target(s). Conditions are usually temporary, harmful and require certain actions to be taken in order to remove them sooner.
If more than one condition affects a character, they are all applied. Certain effects can’t be combined, thus the most severe effect supersedes the others.
Although death is a condition itself, it usually removes most of the other afflicting conditions.
The only condition that can be removed without magical aid or by resting is invisibility.
Complete list of conditons in DDO
Some quests and areas occur in a special environment that changes the rules how your character behaves. Like most conditions, environmental effects often appear on your buff bar. Unlike conditions, environmental effects cannot be removed, unless you leave the location. See Environmental effects.
PnP Dungeons and Dragons
Unlike in DDO, conditions in PnP D&D are permanent. Only if something specially states its removes a certain condition, only then the condition is removed.