This upanishad is based on shukla yajurveda. Below is my summary of its teaching.
Its shaantipaaTha describes poorNa (whole). “whole” is source of all and everything is “whole”. If “whole” is taken out from “whole”, whatever remains is “whole” too.
That which resides within heart inside body, is nitya (eternal) and aja (unborn, without beginning). All five elements – pruthivee (earth), apaha (water), teja (fire), vaayu (air), aakaasha (space) – are its body but those elements are unaware of it.
All four subtle layers of mind – manas (mind, sensory perception), buddhi (intellect, judgmental), ahamkaara (self awareness, ego), chitta (memory storage) – are its body but those layers are unaware.
avyakta (unmanifest), akshara (indestructible), mrutyu (death) – are its body but they are unaware.
That is aatman (soul). 
yogi (ascetic) is always focused on aatman (soul which is subtler than self awareness) and carries out all worldly deeds. 
One must dissolve aatman (soul) into paramaatman (supreme soul) and be at peace eternally. 
One who goes beyond ahamkaara (self awareness or ego) finds root of all. 
When one is free from vaasanaa (desire), that state is moksha (liberation) or mukti. 
One who finds brahman (supreme being) in everything and everywhere, gets free from desires. 
Everything from brahmaa (first being in the cosmos) to stamba (grass) has end. 
Everything is perception of aatman (soul). When one is free from that perception, one becomes poorNa (whole), advaita (non-dual) and parambrahma (supreme being). 
pralaya (annihilation) makes existence into an ocean of “whole”. 
chitta (memory storage) is the cause of vikalpa (alternative, division from “whole”). So, one must meditate upon paramaatman (supreme soul). 
brahman (supreme being) that has form of satya (truth), and is covered by maayaa (material world) and is known as tat (that). 
One that has false perception of existence and called yaha (I or self), is known as tvam (you). 
Act of understanding tattvamasi (that is you or you are that) is shravaNam (study) and thinking about it is mananam (reflection). 
Making chitta (memory) in ekataanatvam (resonance) with shavaNam (study) and mananam (reflection) is called nididhyaasanam (profound and repetitive meditation). 
When chitta (memory) becomes stable, it is called samaadhi (transcendental meditation). 
samaadhi (transcendental meditation) dissolves all millions of karma (deeds) accumulated. 
samaadhi (transcendental meditation) dissolves desires completely and all sacred deeds, sins are destroyed. 
One with state of uparatim (indifference) is called sthitaprajna (steadfast intellect) whose dissolved desires are not born again, who stays in bliss always. 
One who becomes one with brahman (supreme being), and whose tendencies are indifferent and have become chaitanyam (consciousness); those tendencies are called prajnaa (wisdom). One with eternal “wisdom” is called jivanmukta (liberated). [43-44]
jivanmukta (liberated) does not have ego, does not identify with body and senses, does not have any attachment, does not differentiate soul and supreme soul, does not find difference between supreme soul and material world, is always equal towards respect and hate. [45-46-47]
Deeds of dream vanish when one wakes up. Similarly, all millions of karma (deeds) are dissolved when one is enlightened with equality of brahman (supreme being) and aatman (soul). 
One must give up destiny by giving up self identification with body. 
Everything is advayam brahman (non-dual supreme being).