सर्गाणामादिरन्तश्च मध्यं चैवाहमर्जुन। अध्यात्मविद्या विद्यानां वादः प्रवदतामहम् ॥

sargāṇām ādir antaśh cha madhyaṁ chaivāham arjuna adhyātma-vidyā vidyānāṁ vādaḥ pravadatām aham

Of creations the beginning and the ending, and also the middle am I, O Arjuna. Of sciences the science concerning the Self; the speech of orators I.

In plain words: Krishna tells Arjuna that he is the beginning, middle and end of everything that is made. Of all kinds of learning, he is the knowledge of the Self, our true inner being. In people who speak well, he is the reasoning in their speech.