In plain words
The moment a thought rises in us, we must make the effort to destroy it. Bad thoughts, given more spirit and encouragement, keep welling up higher and higher. When enemies come into our fort through a tunnel, only by cutting off each head and casting it away do we leave these warriors no way into the fort. In the same way, the moment our enemies, the bad thoughts, come into the heart one by one, we must make the effort to cut them down. Through these bad thoughts, man's mighty power simply drains away. Through such thoughts, humanness is being turned to soot. So we must see to it that, right from the start, bad thoughts are not let into our heart even for this moment. The moment these bad thoughts get in, our sight, our tongue and all our senses follow them, worked up.
In the same way, good thoughts enter all the senses, and each of them carries out its own work. A small example. Speech too is very powerful. When we put this power to good use, it swells with great joy. When it joins with bad thoughts, our life falls into misery. There is a mantra, "Vedo svarūpo'ham." The Veda is my very form. Śivo'ham. I am Śiva. Ahaṃ brahmāsmi. I myself am Brahman. Tat tvam asi: I am That itself. We should not take these sayings of the Upaniṣads and of the Vedas as our basis and read "Vedo svarūpo'ham" to mean "I myself am the form of the Veda." These sacred sayings and sūtras of the Veda were quoted only so that we may recognise the principle of Brahman, not for us to strut about in the Ahaṅkāra of "I myself am Brahman." So every saying has what are called the śakti-vākya, the śakti-vṛtti and the udāra-vṛtti. All the good sayings told in the Veda and in the Upaniṣads are given simply in an uplifting spirit. That is, they came into being to give the mind the enthusiasm it needs and to bring it encouragement. That is, when we are together with ten people, some strength rises in us to fight with others. How? The courage that says, "There are nine people behind me," gives this man spirit and encouragement. When he stands alone, it brings discouragement.