In plain words
I am the rod of those who discipline, the policy of those who seek victory, silence among secrets, and jñāna among those endowed with jñāna.
Word for word
The rod of those who subdue I am; the policy I am of those seeking victory; and silence indeed I am of secrets; the jñāna of the knowers [am] I.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
दण्डः दमयतां दमयितॄणाम् अस्मि अदान्तानां दमनकारणम् । नीतिः अस्मि जिगीषतां जेतुमिच्छताम् । मौनं चैव अस्मि गुह्यानां गोप्यानाम् । ज्ञानं ज्ञानवताम् अहम् ॥ ३८ ॥
In plain words
Among those who subdue — the tamers — I am the rod, the means by which the unruly are brought into order. Among those bent on victory — those who wish to conquer — I am strategy. And among secrets — the things that must be kept guarded — I am silence itself. In those who possess knowledge, I am the knowing.
Close to the Sanskrit
I am the rod of those who subdue — of the subduers — the means of taming the untamed. I am policy of those who desire victory — of those who wish to conquer. And I am silence itself of secret things — of things to be guarded. I am the knowledge of those possessing knowledge.