Attunement is a characteristic of Rune-keepers that improves their ability to either do damage or heal at the expense of the other. The more battle-attuned the Rune-keeper is, the more damage their attacks will do, but the less healing they will be able to do. The more healing-attuned the Rune-keeper is, the more healing they will output, but the less damage they will be able to do.
Attunement is measured on a scale of 1-18, 1 being fully battle-attuned and 18 fully healing-attuned. An attunement of 10 is neutral (neither battle- nor healing-attuned). When Rune-keepers use skills, their attunement dynamically adjusts from neutral to a maximum of nine battle-attunement (which appears as red marks on the left of the attunement panel), or to a maximum of nine healing-attunement (which appears as green marks on the right of the attunement panel).
Some skills will battle-attune the Rune-keeper, moving attunement towards the left on the panel, while others will healing-attune the Rune-keeper, moving attunement towards the right on the panel. Other skills will neutral-attune by moving attunement towards the center of the panel. Finally some skills will return to neutral attunement, automatically moving attunement all the way to the middle regardless of current attunement.
It's important for Rune-keeper's to carefully monitor their attunement. Some encounters will warrant a Rune-keeper to go either fully battle-attuned or healing-attuned, but other fights may warrant the Rune-keeper to stay closer to neutral in order to either do damage or heal on short notice.
For more information, see Item:Rune-keeper -- Attunement.