Whenever there is decline of righteousness, O Arjuna, and rise of unrighteousness, then I manifest Myself.
In simple words
Krishna makes a promise to Arjuna — and to all of humanity: "Whenever goodness weakens and evil grows stronger in the world, I appear. I manifest myself."
4.7 यदा यदा whenever? हि surely? धर्मस्य of righteousness? ग्लानिः decline? भवति is? भारत O Bharata? अभ्युत्थानम् rise? अधर्मस्य of unrighteousness? तदा then? आत्मानम् Myself? सृजामि manifest? अहम् I.Commentary Dharma is that which sustains and holds together. There is no proper eivalen for this term in the English language. That which helps a man to attain to Moksha or salvation is Dharma. That which makes a man irreligious or unrighteous is Adharma. That which elevates a man and helps him reach the goal of life and attain knowledge is Dharma that which drags him and hurls him down in the abyss of worldliness and ignorance is Adharma.
Contemporary scholarly and practical interpretations for modern seekers.
This interpretation draws on a specific tradition and may not represent the view of any single school. For authoritative guidance within a specific tradition, seek a qualified teacher.
Modern
Whenever dharma declines and adharma flourishes, the Divine takes birth upon this earth. In the opening discourse of the Gita's first chapter, the meaning of dharma has been expounded at length. Dharma is a sacred truth, and only through its observance can society be sustained and preserved. When the majority of people abandon dharma, the world falls into the grip of those who walk as beasts in human form. In such times, families bound by mutual care and joy become invisible; the noble and elevated life that adorns humanity vanishes from sight. In these dark ages of history, a great soul appears in society, striving to elevate the moral and spiritual condition of human life. This work cannot be accomplished merely by advancing existing moral values—the destruction of wickedness must also occur.
For this sacred purpose, the infinite Supreme Being sometimes assumes the limitations of body and form, manifesting upon the earth. Like a great lord of wealth who occasionally inspects his vast estate and sets it in order, taking up weapons and tools, standing among laborers toiling in the sun—yet never forgetting his sovereignty—so too does the Lord of all creation, the Bhagavan, assume a mortal form. Dwelling among unrighteous humanity, remaining untouched by their transgressions, He labors ceaselessly to draw them from adharma and guide them toward the path of dharma.
In this divine descent, one truth is made clear: though the Bhagavan assumes a body, He does not lose His freedom and sovereignty. Though He dwells within limitations, He does not become enslaved by them.
For what purpose...