Bhagavad Gita Verse 5, Chapter 4

Shree Bhagavaan uvaacha:
bahooni me vyateetaani janmaani tava chaarjuna |
taanyaham veda sarvaani na tvam vettha parantapa || 5 ||
 
Shree Bhagavaan said:
You and I have have undergone many births, O Arjuna. I know all of those (births), but you do not know them, O scorcher-of-foes.
 
bahooni : many
me : mine
vyateetaani : have happened
janmaani : births
tava : your
cha : and
arjuna : Arjuna
taani : those
aham : I
veda : know
sarvaani : all of them
na : do not
tvam : you
vettha : know
parantapa : scorcher-of-foes
 
In this shloka, Shri Krishna begins to reveal his divine nature to Arjuna. Previously, Arjuna raised a doubt as to how did Shri Krishna convey the Gita teaching to the sun. Shri Krishna responds by pointing out the similarity as well as the difference between his birth and Arjuna's birth. The similarity is that both Arjuna and Shri Krishna have taken many births in the past.
 
However, the difference is that Shri Krishna had the knowledge and memory of all his prior births, whereas Arjuna did not. Normally, human beings have such strong identification with their body that it restricts their ability to remember what occurred in another body. We even tend to forget events associated with our own body with the passage of time, including simple things like where we kept our keys in the morning.
 
So if Shri Krishna could recall what happened in all his births, he did not have any identification to his body, and therefore he is not restricted by its limitations. He was speaking as an enlightened being who realized the eternal essence and has dropped identification with his body. But there is more to Shri Krishna than just this aspect. He will reveal more in the next shloka.
 
Footnotes
1. How does the Gita treat the topic of rebirth? Our vaasanaas force us to take a birth in a body. If we chose to use the birth to exhaust those vaasanaas, then the cycle of rebirth is broken. But if we chose to use the birth to accumulate more vaasanaas, then we will be forced to take another birth to exhause the newly acquired vaasanaas. Only by cleansing our vaasanaas can we break the cycle of rebirth or "samsaara".