Purport

Seeing the ferocious manifestation within the universal form of Shri Krishna, containing thousands of burning mouths and which is consuming the whole world, terrified Arjuna asked who you are?
And then that ferocious form within Universal form of Shri Krishna replied. "Time I am". I am the one who destroys the world and the end of their life cycle. I have manifested here at the field of Mahabharata to consume all those who have completed their life cycle.

The ferocious form in Shri Krishna's universal form further explained to Arjuna that all those assembled in the filed of kurukshetra will die even if Arjuna is not fighting.

This Shloka is one of the most mystic Shlokas from Shrimad Bhagvad Gita and refers to one of the core concepts of Santan Dharma, the law of karma and the concept of kal(The time). The word kala means Time that binds as well as disintegrates, the embodiment of Wisdom and the Perennial Principles established by Resplendent Lord. The word Kaala derived from the root kla(कल), as that which binds, severs and provider of all desires and popularly known as the Resplendent Lord used by Bali when he addressed Indra – “O lndra, you are praising yourself before me who is bound by the shackles of Time. The Time, verily, is Sri Hari. He is the Person dark in color who binds the people of the world, and once having bound and pulling them stands out terrible as Time”. “Sri Vishnu as the symbol of Time draws towards Himself the mind of the people”, thus in Bhagavat Purana.

kala (the time) hence is a primeval factor or to say the component of the universe and one of inner potency of Bhagwan Shri Krishna. This is the kala(the time) by which the whole propagation of the universe binds with. With time the whole universe moves into one irreversible cycle.

In scientific terms, this kala(the time) is the same as the concept of time in the theory of relativity, where time is one dimension of time-space coordinate. Everything in this universe exists on time in scale.

The other fact which has been declared in this Shloka is that kala is deeply attached to the karma of living being. Hence something done in the past creates a situation of kala that is the state of time and space which can't be undone. Therefore, Shri Krishna says, everybody who assembled in Kurukshetra has been driven by their underlying Karma and many of them will perish no matter if Arjuna participates in the war or not. Arjuna’s participation is part of his karma and not going to alter the destiny which others have created from themselves and assemble in kurukshetra to perish.