In plain words
Yama and niyama mean restraining the inner and outer senses and bringing them under control. Restraining the senses and doing tapas. What is tapas? It is not standing on your head with your feet in the air and taking a vow. Tapas is the purity of thought, word and deed. To speak out what has arisen in the heart, and to carry out with the hand what has been spoken through the tongue: that alone is tapas.
This is what they called trikaraṇa-śuddhi. When you are with yama and niyama, in the purity of thought, word and deed, God, who is Satya, appears before you. That is why: Satyaṃ jñānam anantaṃ brahma. Satya is Jñāna, Jñāna is the infinite. The infinite is Brahman. What has taken on the infinite form is Brahman. So Satya is God's form itself; but the man of today takes worldly truth for Satya, forgets the Satya of the highest reality, sinks into a kind of path of Ajñāna, sinks into blindness, and cannot see the light. Worldly truth keeps on changing.
Brahmacarya, gṛhastha, vānaprastha, sannyāsa: these are the Dharmas of the āśramas. But in these four Dharmas of the āśramas, the forms and names of brahmacarya, gṛhastha, vānaprastha and sannyāsa may change. All that undergoes this change is worldly truth. The truth of the highest reality is that the one present in all four is one and the same person. But the changes are different. What changes is truth. What does not change is Satya. What does not change is itself the foundation. That is Satya. Satyaṃ nāsti paro dharmaḥ. On the foundation called Satya the worldly Dharmas are built. You may paint the walls any number of colours. You may put any number of decorations on the walls. You may give the wall any number of names. But the foundation cannot be changed. That is Satya. What rests on that Satya is Dharma. What holds Satya is Dharma. What chooses Satya is Dharma. What chooses Dharma is Satya. What is the foundation of everything is Satya. Today we are forgetting such a principle of Satya. The ancient glory, the culture of the ancient Veda: "Satyaṃ vada dharmaṃ cara." Today you cannot find among men even one in a hundred, even one in a crore, who speaks Satya. Once a year they perform the Satyanārāyaṇa vrata. But for the rest of the year it is all Asatyanārāyaṇa vratas! Sometime, once in our lives, we speak Satya. All our life we live a life of Asatya.