In plain words
Man today forgets himself and is trying to know the world. Man knows everything, but does not know who he himself is. Without knowing himself, what is the use of knowing everything? So first of all he must try to know himself. For knowing oneself, Prema alone is the basis. Prema alone is the chief thing. Prema alone is the measure. That is why the Upaniṣads compared Buddhi to a bird. This bird has two wings and a tail. Its head itself they named śraddhā. The right wing itself they called Ṛta; the left wing itself they called Satya. The tail itself they called yoga.
In short, Buddhi means what is joined with śraddhā, Ṛta, Satya and yoga. Śraddhā means keen interest. Without this śraddhā, man will not take up even the smallest task. One who has śraddhā can achieve any great task whatever. That is why Vedānta said, 'Śraddhāvān labhatē jñānaṃ'. Where there is śraddhā, however hard, every man sets out to achieve anything, however hard it may be. Without śraddhā he cannot win success even in the smallest task. So Buddhi is what is joined with śraddhā.
Next, Ṛta. Ṛta means simply that which is joined with the three karaṇas. The single form of all three, Manas, speech and deed, is Ṛta itself. In short, Buddhi is what is joined with this very trikaraṇa śuddhi. Next, Satya means what belongs to Prakṛti. To speak in words what you saw as you saw it, what you heard as you heard it, what you did as you did it: this itself is Satya. But we must try to recognise the difference between Satya and Ṛta. Satya tells in detail only of what has happened at that moment. But Ṛta recognises the Satya that does not change in the three times. When you see a person, he may be wearing a bush coat, he may have on pants. For that moment, it is indeed Satya. Tomorrow he may wear a pyjama and kurta; for tomorrow, that is Satya. What you can recognise at that moment is Satya. What does not change even in the three times is Ṛta. So Buddhi is what is joined with śraddhā, Ṛta and Satya.
Next, the tail, yoga. What does yoga mean? Joining, uniting. It is what is joined with an evenness that unites one thing with another. It is what comes under the cittavṛttis; that is why Patañjali said, 'Yōgaḥ cittavṛtti nirōdhakaḥ'. It is what restrains the cittavṛtti. Only when he has restrained that cittavṛtti can he travel to any place whatever, at any time whatever.