We hear this on strings alone at the very start of the prelude to the third act. This prelude is music of utter desolation. The simple motif of Serving appears throughout the first scene of this act, in which the penitent Kundry and the elderly Gurnemanz greet the stranger.
The falling fifths might be an allusion both to the Prophecy motif and to the falling fourths of the Bells motif. The second part of the theme (a) seems to allude to the subsidiary motif of Nature (#16), which appeared earlier in the contexts of Nature's Healing and in the seduction music of the Flower Maidens.
The motif of Serving returns at the first words of Amfortas (Ja, Wehe! Wehe!) in the final scene. Here it seems to associate the weariness of Amfortas, waiting for Parsifal, with the weariness of Parsifal seeking Amfortas. So an alternative name for this idea would be weariness.
Albert Lavignac called this motif the desert and von Wolzogen called it desolation.
Serving