manushyaanaam sahastreshu kashchidyatati sidhhaye |
yatataamapi siddhaanaam kaschinmaam vetti tattvataha || 3 ||

Among thousands of people, perhaps one strives for success. Among those successful strivers, perhaps only one knows me in essence.
manushyaanaam : among people
sahastreshu : thousands
kashchit : perhaps one
yatati : strives
sidhhaye : success
yatataam : among those strivers
api : only
siddhaanaam : successful
kaschit : perhaps one
maam : me
vetti : knows
tattvataha : in essence
Shri Krishna introduced this chapter by saying that he will reveal his true nature to Arjuna, which comprises both knowledge and wisdom. He also said that by knowing this, nothing else will remain to be known. Given the powerful nature of this knowledge, one would hope that everyone would seek this knowledge rather than running after new sources of knowledge everyday. But this is not the case. Here, he says that those who seek this knowledge are extremely rare in this world.
Let us look at the language used in the shloka. The word “sahasra” literally means thousands, but used in this context, it means innumerable or an extremely large quantity. So the shloka then reads: out of innumerable people, only one strives for success in liberation. Out of those few strivers, only one knows Shri Krishna in his essence.
Here, we can raise a doubt and say: what about the millions of people who go the the temple everyday? They worship Ishvara in his numerous forms. Many people observe fasts. Many people have a little temple in their homes. Many people conduct religious ceremonies where they donate food and other items. So how can it be said that only few people know Ishvara?
This doubt can be answered by looking at the last part of the shloka where Shri Krishna says that he has to be known in his essential nature. Most of the people mentioned earlier worship Ishvara, but they tend to have a partial understanding of what Ishvara really is. That is why Shri Krishna devoted almost a third of the Gita in describing his essential nature.
Having introduced the topic of Ishvara, Shri Krishna begins the main portion in the next shloka.