Sanskrit
यद्यदाचरति श्रेष्ठस्तत्तदेवेतरो जनः । स यत्प्रमाणं कुरुते लोकस्तदनुवर्तते ॥
yad yad ācarati śreṣṭhas tat tad evetaro janaḥ sa yat pramāṇaṃ kurute lokas tad anuvartate
Translation
Whatsoever a great man doeth, that other men also do; the standard he setteth up, by that the people go.
Annie Besant · The Bhagavad-Gita, translated by Annie Besant (4th ed., 1922) — Wikisource · Public Domain
Explanation
A general sociological observation that supplies the mechanism behind verse 20’s lokasangraha: the wise act as they do specifically because others imitate them. It’s a plain statement of what becomes, in the next four verses, Krishna’s justification for his own conduct — he’s about to apply this exact principle to himself.