When damage is inflicted upon a player, it shows up as red numbers hovering over the player. When a skill was used to inflict the damage, the skill icon is displayed next to it. For ~10 seconds the Damage Monitor shows the player the last x skills (depending on box size) which damaged them. The skill icons behave like the ones in the Skill Bar, if a player dies the last x skills will be displayed until they are resurrected.
In your client option you can:
- disable the damage monitor
- choose the size of the red numbers