Krishna tells Arjun (2:25) that this( atma /soul) is said to be unmanifested, inconceivable and unchangeable, once you are aware of this, there is no need to grieve for the physical body. Krishna further says that (2:28) all the beings are unmanifest before their birth, they manifest between their birth and death and once again unmanifest after their death.
Many cultures use the ocean and wave analogy to explain the same. The ocean represents unmanifest and wave represents manifest. Waves arise from the ocean for some period of time and they manifest in different sizes, shapes, intensities etc. Our Indriyas(senses) can only sense the manifest i.e. waves. Finally waves merge back into the ocean from where they arose.
Similarly, a seed holds the potential to grow into a tree. In the seed, the tree is present in its unmanifested form. It becomes manifested when it starts growing into a tree. It ultimately dies after producing many seeds.
Manifested is that which Indriyas , with their limited capabilities, can sense. Even the scientific instruments are for the enhancement of the capabilities of our Indriyas. Microscope/telescope is to enhance the ability of the eyes to magnify. X-ray machine is to enable eye to see things in different frequencies of light.
Krishna says this(unmanifest) is inconceivable; which means that our Indriyas even aided by scientific instruments won't help us to perceive this. The mind is incapable of conceiving the unmanifest, as the mind is a combination of Indriyas .
Like all of us, Arjuna identifies himself with the human body, as he has no realisation or experience beyond that. Krishna tries to bring about a paradigm shift in Arjun's thinking by enlightening him about the unmanifested. It took the lord himself to make a scholar like Arjuna understand this and we are no exception.
Source - Daily World