This upanishad is from kruShNa yajurveda. Bhagavaan dattaatreya answered saankruti’s curiosity and thus this upanishad is born.
aum saha naavavatu. saha nau bhunaktu. saha viryam karavaavahai. tejasvinaavadheetamastu maa vidviShaavahai. aum shaanti shaanti shaantihi.
(God) protect both of us (guru and disciple). Nourish us together. We both gather power together. Our knowledge be brightened. We never envy each other. Peace, peace and peace. [shaantipaaThaha]
Meaning of avadhootam: ‘a’ signifies akshara – indestructible brahman , ‘va’ signifies vareNyam respected, ‘dhoo’ signifies dhoortam false worldly bondage, ‘ta’ signifies tattvamasyaadi – that is you – feelings. So, a person with all these four qualities is called avadhootam. [1-2]
avadhoota does not believe in worldly social divisions and casts. 
S/he has beloved brahman as head, joy as right side, cheerfulness as left side and bliss as center aatman (soul). 
Only sacrifice or leaving everything behind brings immortality. 
avadhoota is continuously focused on aatman and that is his/her ashvamedha yajna (horse sacrificing fire ceremony). S/he offers all outward/inward energy channels to super consciousness. This is mahaa yajna or mahaa yoga. 
Sun extracts all types of chemicals but is not affected by those. Similarly, yogi is not affected by any worldly deeds – right or wrong. S/he remains calm in any condition. [9-10]
I (dattaatreya) do not do any worldly activities. I do not do any dhyaanam or nididhyaasanam (final step of mukti (liberation)). [12-19]
Humans behave as humans because of their habits and attachment. Until past karmas to be experienced by present body are destroyed, one cannot be free from that attachment. [20-21]
When mind is disturbed, intellect becomes unstable. One needs samaadhi in that condition. 
avadhoota never does any karma even though his/her body keeps doing prayers, mind keeps meditating. S/he is only an observer of karmas. 
Only om (brahman) is the truth and only truth. That is upanishad (teaching).