Bhagavad Gita Verse 20, Chapter 14

gunaanetaanateetya treendehee dehasamudbhavaan |
janmamrityujaraaduhkhaurvimuktomritamashnute || 20 ||
 
Having gone beyond these three gunaas, the creators of the body, the body dweller is freed from sorrow of birth, death and old age, and attains immortality.
 
gunaan : of the gunaas
etaan : these
ateetya : gone beyond
treen : three
dehee : body dweller
dehasamudbhavaan : creators of the body
janmamrityujaraaduhkhaihi : sorrows of birth, death and old age
vimuktaha : freed
amritam : immortality
ashnute : attains
 
Shri Krishna now conveys the gist of the fourteenth chapter in this shloka. He says that Prakriti, which is comprised of the three gunaas, creates this entire universe, including our body. As long as the individual soul, the jeeva, the body dweller identifies with this body and gives reality to it, he experiences sorrows throughout his life. Once the jeeva stops identifying with the body which is the product of the three gunaas, he attains liberation and becomes immortal.
 
This shloka also connects the main theme of the prior chapter to this chapter. In the thirteenth chapter, we learnt that ignorance of our true nature creates an entity called the Purusha, which in essence is nothing but a bundle of unfulfilled desires. Now here, we learn that these unfulfilled desires, also known as avidyaa, ignorance or maaya, attract a combination of sattva, rajas and tamas that becomes the physical body. Only when the Purusha severs his connection with this body by rising above the three gunaas, he realizes his true nature as the eternal essence.
 
Immortality in this context does not mean physical immortality. The body follows the laws of nature. Everything in nature has to perish, including our physical body. From the perspective of Prakriti, the body will die. But the dehi, the body dweller, having gone beyond the three gunas, having stopped identifying with the three gunas, knows that he is untouched and unaffected by the laws of Prakriti. For him, the modifications of the body such as old age and disease are as if they are happening to some third party. This ultimate freedom from the influence of the Prakriti is what makes him immortal.