In plain words
That is exactly why they named it the aśvattha tree. The kings of old all performed a special sacrifice called the aśvamedha yāga. When we grasp the inner meaning of these two words, aśva and medha, it becomes quite clear to us what the aśvamedha is. Medha means Buddhi. Aśva means what moves. Aśvamedha means the moving Buddhi. The horse is just the image of this moving Buddhi. In this aśvamedha sacrifice, whoever could catch this horse, they alone were called the wise. Who would catch it? Who would subdue it? Whoever could restrain it to some extent, they alone, they said, were the truly steadfast. So the wise one is the very person who curbs our senses and brings the senses under control. Within this teaching, in its outer meaning, the kings of old went on showing their power: that in this physical world whoever could catch this horse was the wise one, and that he alone had the right to fight them. Here the very harmony of the inner meaning and the outer meaning is the real truth. So today every human being, too, must catch this horse and tie it up. Only then do they become steadfast. However many yāgas, however many yajñas, however many sacrifices they perform, however many books they recite, those who cannot restrain this have done nothing but make all of it a waste.
This word mātraḥ has another meaning too. That is, the senses are what fix the limit. God has laid down for each sense one limit only. The eye can see, but it cannot hear. The mouth can speak, but it cannot see. In this way He set for each sense its own fixed rules. Whoever keeps properly to the rules laid down for it, they alone become ones who have obeyed the command of God. Whoever transgresses these rules is subject to its punishment. You must use each sense according to what has been fixed for it, and try to protect it. He gave the nose its appointed task. To take in and send out the breath and so guard the health. To tell fragrance from foul smell and take in nothing but the fragrance: carrying out appointed actions such as these is precisely the work of this nose. But, unfortunately, transgressing what has been fixed, using this nose besides to sniff snuff and to breathe in foul smells: by doing actions of this kind, against its nature, a person becomes one who has transgressed the rules laid down.