Tuḍīʾu is the active imperfect of aḍāʾa yuḍīʾu, Form IV from ḍ-w-ʾ (ض و ء). Al-ẓalām is its object, giving the meaning “she illuminates the darkness.” Bi-l-ʿishāʾ specifies the evening setting.
Ḍiyāʾ (light) and ẓalām (darkness) are semantic opposites. Placing brightness against a dark background creates meaningful contrast and makes the imagined illumination more striking.
The verse's rhetoric is not limited to the comparison introduced by ka-annahā. The active verb tuḍīʾu first animates the scene, and the second hemistich provides the concrete model of a monk's light.