Jesus said to them, “Most assuredly, I say to you, before Abraham was, I AM.” -- John 8:58 (NKJV)

Out of context it seems like Jesus claim to exist before Abraham was born, but a few verses before Jesus explain:

Jesus answered, “If I honor myself, my honor is nothing. It is my Father who honors me, of whom you say that He is your God. Yet you have not known Him, but I know Him. And if I say, ‘I do not know Him,’ I shall be a liar like you; but I do know Him and keep His word. Your father Abraham rejoiced to see my day, and he saw it and was glad.” -- John 8:54-56 (NKJV)

In other words, Abraham saw (a vision) of the future ("my day") that contained Jesus. The author of Hebrews also claim that Abraham saw visions of the future:

By faith Abraham obeyed when he was called to go out to the place which he would receive as an inheritance. And he went out, not knowing where he was going. By faith he dwelt in the land of promise as in a foreign country, dwelling in tents with Isaac and Jacob, the heirs with him of the same promise; for he waited for the city which has foundations, whose builder and maker is God. -- Hebrews 11:8-10 (NKJV)

In "prophetic language", prophets sometimes refer to the future as the past or would say that someone existed or did something before it happened in our timeline. This is because prophecies originate from God who is outside our timeline.

Therefore, an alternative interpretation is that before Abraham was born, there was already a prophecy that Jesus would be born and Abraham saw that vision. Like many other prophecies that declare a future event as the past, Jesus quoted a prophecy written in past tense which was about a future event that was being fulfilled by him in their presence.