This is how dragons mark their territory.

Dragonmarks are unique to Eberron, and with their recent
addition to DDO, might pose a problem for new players... should one take
dragonmark feats or not?

"However, when used with certain builds, which capitalize with
a dragonmark's abilities, dragonmarks can make a build much more powerful."

Read the full guide at Ten Ton Hammer's DDO site.