In plain words
These senses are subtler than the body. In the body there are sound, touch, form, taste, smell and the rest, and there are wind, bile, phlegm and the rest; yet the nature of the senses, holding itself equal to all of these, keeps wielding its power over the body. Only when the senses touch their objects do pleasure and pain, cold and heat arise. Without objects, the senses do not stay still even for a moment. Contact with objects is the very chief duty of the senses. For the objects, it is not possible to regard the senses merely as one, to choose them as divine and to make them … nobly.
The senses have a fine name. Mātrāḥ is a name for them. '… iti mātraḥ.' That is, the power of these senses is just this: they fix measurements of a kind. In what way do these senses take these measurements? They have the authority to decide, 'This fruit is sweet,' or 'It is sour.' Which sense measures this? That is none other than the tongue. This tongue measures the good and bad of things and hands them on to the world on its authority. 'This picture is wonderful.' 'It is ugly': it has the measure to judge the wonders of a picture. This measuring rod is the eye alone. And all the senses, too, are just like this. The nose is the measuring rod that decides, 'This is a foul smell, this is a fragrance.' The ear is the measuring rod that decides, 'This is a sweet sound, this is a harsh one.' Precisely because these senses have the authority to decide, they are named mātrāḥ.
In the Kaṭha Upaniṣad they laid down that, for the chariot that is the body, the senses are like horses. What is the reason for giving these senses the name of horses? A horse means that which by its nature is ever on the move: that is a horse. This is a thing everyone knows. Whether a horse is asleep, awake or standing, it keeps moving some part or other of its body. If not its tail, then a leg; if not a leg, then a foreleg or its nose; if not its nose, then its mouth: it keeps moving something or other. It is because its nature is to move that it got the name aśva. In the same way, we see the peepal tree. Yet even when there is no wind, its leaves are always fluttering.