iti guhyatamam shaastramidamuktam mayaanagha |
etadbuddhvaa buddhimaansyaatkritakrityashcha bhaarata || 20 ||

Thus, this foremost secret has been taught to you by me, O sinless one. Having known this, one becomes wise and accomplishes all his duties, O Bhaarata.
iti : thus
guhyatamam : foremost secret
shaastram : science
idam : this
uktam : taught
mayaa : by me
anagha : O sinless one
etat : this
buddhvaa : having known
buddhimaan : wise
syaat : becomes
kritakrityaha : one who has accomplished all duties
cha : and
bhaarata : O Bhaarata
When you ask the question – can you stop working right now and retire, you get a couple of answers. Some people say that they have still so many desires, so many plans to fulfill, that’s why they cannot retire. Other people say that they still have so much to learn from the world, so much knowledge to acquire. Shri Krishna concludes this chapter by asserting that one who has truly understood the teaching of this chapter has accomplished whatever anyone can accomplish in this world, plus he has also known whatever can be known in this world.
Why does he say that whatever has to be known has been covered in this chapter? The highest knowledge to be known in this world is the understanding of three topics. What is the nature of the individual soul (who am I), what is this world and where did it come from, and what is beyond this world (is there a God). Any text that conclusively answers these three questions is termed a shaastra, a science. The fifteenth chapter of the Gita does so, and hence it is worthy of being termed a shaastra.
The method used to reveal Purushottama, the pure eternal essence, is to gradually move from the tangible to the intangible, from the visible to the subtle, from the visible universe to the invisible Prakriti to Purushottama who is beyond both. This method is known as Arundhati nyaaya, the technique of revealing the location of the star known as Arundhati. Here the teacher first points to a tree, then to one of its branches, then to one of its leaves, and then to the star that is right next to the tree. Without doing this step by step revelation, it would not have been possible to reveal the position of the star.
So then, the teaching of this chapter is called the foremost secret. It is secret because such knowledge is not accessible to any of these sense organs. It has to be revealed through a teacher who has had direct experience of the eternal essence. Furthermore, it has to be taught to a student who is straightforward and without sin like Arjuna. Shri Shankaraachaarya goes so far as to say that this chapter summarizes the teachings of all of the Vedic scriptures.