Qadhā refers to a small mote or impurity entering the eye. ʿUwwār concerns the troublesome presence or effect of such grit, while ṭaḥūrān expresses strongly clearing it away.
Together the words convey more than attractiveness: the eyes remain clear of irritating specks. The simile that follows extends this cleanliness into a picture of naturally outlined, attentive eyes. Precision about the vocabulary prevents the verse from becoming a vague compliment.