The Kena Upanishad derives its name from the first word of the text Kena, which means “By whom”. This Upanishad is also referred to as the Talavakara Upanishad, since it belongs to the Talavakara Brahmana of the Saamveda. The Upanishad has 4 sections - the first two are in verse and the last two are in prose. Despite it’s conciseness, the Kena Upanishad is considered one of the more important Upanishads. In fact Adi Shankaracharya has written two separate commentaries on this Upanishad - the Kenopanishad Pada Bhashya and the Kenopanishad Vakya Bhashya.
This Upanishad explains the nature of the Supreme Being. The ears cannot hear it, the eyes cannot see it, the tongue cannot describe it. The nose cannot smell it. The mind cannot even imagine it. The Supreme Being is what gives the ability to hear to the ears, the ability of sight to the eyes, the ability of speech to the tongue, the ability of smell to the nose and the ability to think to the mind. It cannot be known through the senses, because it directs the senses themselves.
The Brahman is infinite and indescribable, hence the mind lacks the capacity to fully understand it. Those who think that they have understood it, have no knowledge of it. Those who think that they do not know Brahman, know it, because they have realized its infinite nature.