Tat Tvam Asi

   Tat 

     The vachyartha of tat, is the jagad karanam ishvara, who is bhagavan. Owner of six bhagas.

ऐश्वर्यस्य समग्रस्य वीर्यस्य यशसः श्रियः।

ज्ञान-वैराग्ययोश्चैव षण्णां भग इतीरणा॥

These are: all knowledge, jnana; total dispassion, vairagya; the capacity     to create, sustain, and resolve, virya; absolute fame,yasas; all        wealth, sri; and overlordship, aisvarya.

The lakshyartha, is satyam brahma. The reality of ishvara, which is satchitananda.

What is satchitananda brahman?

Tvam 

Tvam in general refers to jiva. But when we look at oneself atma, considered to be jiva, atma is 
    sakshi chaitanyam, chit, who is witness consciousness, of 3 states of waking dreaming sleeping,
   confused with the panchakoshas, and with the dehatrayam (sthula sukshma karana sharira), but
   sakshi chaitanyam is pure consciousness, and being witness to space and time, is limitless in terms
   of space and time, and self existent.

This sakshi chaitanyam is brahman, the jagad karanam.

This is mahavakya, wherein the bhagavan is equated with jiva as one and the same. That is
   understood by taking the lakshyartha. Lakshyartha refers to the one truth, which is non dual
   satchitananda brahman, who is alone atma.

Doership, ahamkara is sublated by this knowledge. It is a mere ornament for the survival of the
    body mind, which itself is ishvara srishti, wherein the total maya sharira is pervading the individual
    body mind as well.

So let there be no doubt with respect to the fact of atma being brahman, and deha and the
    universe being mithya karyam. The seeing doing is ishvara anugraha, and maya anugraha, whole
   jagad can be appreciated as an effect, and oneself understood to be non separate reality or
   satyam.