Bhashya
यन्तं गच्छन्तं य एवायं शब्दः पश्चात् पृष्ठतः अनूदेति, अध्यात्मं च जीवनहेतुः प्राणः- तमेकीकृत्याह ; असुः प्राणो जीवनहेतुरिति गुणस्तस्य ; फलम्- सर्वमायुरस्मिँल्लोक एतीति- यथोपार्त्तं कर्मणा आयुः कर्मफलपरिच्छिन्नकालात् पुरा पूर्वं रोगादिभिः पीड्यमानमप्येनं प्राणो न जहाति ॥
In plain words
The sound that trails behind a walking person is united here with prana, the inner life-cause. 'Asu' (breath) as the life-cause is its defining attribute. The fruit of this meditation is that one attains his full lifespan in this world: even if afflicted by illness before his natural time, prana does not desert him.
Close to the Sanskrit
The sound which follows behind a moving, walking person—subjectively, prana is the cause of life; having unified that, he speaks. Asu is prana, the cause of life—that is its attribute. The fruit: 'he attains all lifespan in this world'—even when afflicted by diseases etc. prior to the time of life limited by the fruit of karma, prana does not abandon him.