In this mantra of Śrī Īśopaniṣad, the devotee prays to the Lord to rectify him from within his heart. To err is human. A conditioned soul is very often apt to commit mistakes, and the only remedial measure to take against such unintentional sins is to give oneself up to the lotus feet of the Lord so that He may guide one to avoid such pitfalls. The Lord takes charge of fully surrendered souls; thus all problems are solved simply by surrendering oneself unto the Lord and acting in terms of His directions. Such directions are given to the sincere devotee in two ways: one is by way of the saints, scriptures and spiritual master, and the other is by way of the Lord Himself, who resides within the heart of everyone. Thus the devotee, fully enlightened with Vedic knowledge, is protected in all respects.
Vedic knowledge is transcendental and cannot be understood by mundane educational procedures. One can understand the Vedic mantras only by the grace of the Lord and the spiritual master (yasya deve parā bhaktir yathā deve tathā gurau [ŚU 6.23]). If one takes shelter of a bona fide spiritual master, it is to be understood that he has obtained the grace of the Lord. The Lord appears as the spiritual master for the devotee. Thus the spiritual master, the Vedic injunctions and the Lord Himself from within-all guide the devotee in full strength. In this way there is no chance for a devotee to fall again into the mire of material illusion. The devotee, thus protected all around, is sure to reach the ultimate destination of perfection. The entire process is hinted at in this mantra, and Śrīmad-Bhāgavatam (1.2.17-20) explains it further:
Hearing and chanting the glories of the Lord is itself an act of piety. The Lord wants everyone to hear and chant His glories because He is the well-wisher of all living entities. By hearing and chanting the glories of the Lord, one becomes cleansed of all undesirable things, and then one's devotion becomes fixed upon the Lord. At this stage the devotee acquires the brahminical qualifications, and the effects of the lower modes of nature (passion and ignorance) completely vanish. The devotee becomes fully enlightened by virtue of his devotional service, and thus he comes to know the path of the Lord and the way to attain Him. As all doubts diminish, he becomes a pure devotee.