sadbhaave saadhubhaave cha sadityetatprayujyate |
prashashte karmaani tathaa sacchabdaha paartha yujyate || 26 ||

The word Sat is used in the sense of existence and goodness, and also, O Paartha, the word Sat is added in the sense of an auspicious act.
sadbhaave : Sat as existence
saadhubhaave : Sat as goodness
cha : and
sat : Sat
iti : in this manner
etat : this
prayujyate : used
prashashte : auspicious
karmaani : act
tathaa : also
sat : Sat
shabdaha : word
paartha : O Paartha
yujyate : added
The utterance of Om is used to focus our attention on the action, and the utterance of Tat is used to dedicate the actions and their reward to Ishvara. Even if we do all this, there could be a defect in the way the action is performed. We may have not followed some guidelines, or some external entity may have caused some problem which we may be unaware of. For most of us who are not well versed in the scriptures, is there an easy solution to this problem?
Shri Krishna says that the chanting of the word Sat during the performance of a saattvic action has the effect of removing all the errors and defects of that action. This is why words such as satkarma (good actions) and sadaachaar (good conduct) use sat as a synonym for good. But just purifying an action is not enough. The emotions, the feelings behind the action are equally important. Chanting of the word Sat has the effect of purifying our emotions as well. Words such as sadbhaava (good emotion) and sadguna (good values) illustrate this point.
The real meaning of the word Sat, however,is existence. It is a pointer to brahman, the eternal essence, the one reality. “Naabhaavo vidyate sataha” found in the second chapter of the Gita denotes that Sat, the eternal essence, always exists. It is always complete, without any duality. Therefore, the ultimate goal of uttering Sat is to mentally remove any notions of duality, mentally remove all the upaadhis or limitations, and merge oneself into the one reality, the one eternal essence.