In shloka 3.9, Krishna tells Arjuna,
Aside from action for the purpose of sacrifice, this world is bound by action. Perform action for the purpose of sacrifice, Arjuna, free from attachment.
In doing so, he instructs Arjuna that the only action he should perform should be “free from attachment.” This is known as ‘sacrifice’ because it is the act of renouncing or giving up attachments. Being free from attachment, one has knowledge of Brahman. As Krishna says in the fourth chapter (4.23),
The work of one who is free from attachment, who is liberated, whose thought is established in knowledge and is done only for sacrifice, one’s karma wholly melts away.
The concept of sacrifice comes from the Vedas, where recitation of a mantra was seen as the sacrifice. In the act of reciting, the breath and senses were viewed as the primary ‘offerings’. We can see, in the Upanishads, the elaboration of this idea. Chandogya Upanishad 1.3.3 declares – “one utters speech while neither breathing out nor breathing in”, and Kaushitaki Upanishad 2.5 similarly says – “as a person is speaking, they cannot breathe. At that time, they offer up the breath”. Prashna Upanishad 4.4 even calls inhaling and exhaling the “two offerings”.
About the senses and their role in the performance of sacrifice, Maitri Upanishad (6.10) tells us: “one who does not touch sense-objects is a renouncer” and “sacrificer to the Self.” A ‘sacrificer to the Self’ means being “established in knowledge”, which attachment to the senses deprives us of. So it is understood that one who has knowledge of Brahman or the Self is a ‘sacrificer’ – they have renounced their attachment to Maya.
Krishna also mentions offering up the breath and senses as examples of sacrifice [2]. Attachment to the senses contributes significantly to being entrapped by Maya and the ensuing desire which leads us to chasing the ‘fruits’ of our actions. Being attached, one performs actions with the desire to satisfy the senses through their objects. With the ‘fire’ of knowledge, this attachment is burnt and one discerns Maya, which the senses are part of.
In short, the “action” which Krishna discusses in 3.9, refers to acting with attachment, whereas “sacrifice” is without. The world “bound by action”, therefore, means Maya. In contrast, “action for the purpose of sacrifice”, to give up attachments, is the path of liberation. As Krishna explains in 4.23 – “one who is free from attachment…is liberated”.
[1] 4.26; 4.27; 4.29