When the accumulated merit (the cause of heavenly pleasures) is exhausted they descend to this world. They come and go. They have no independence.
Translation
9.21 Having enjoyed the spacious world of heaven, they return to the world of mortals their merit is exhausted. Thus, those who follow the Vedic rituals and are drawn by desires, come and go.
Meaning
When the accumulated merit (the cause of heavenly pleasures) is exhausted they descend to this world. They come and go. They have no independence. The Dharma of the three Mere Vedic ritual enjoined by the three Vedas. KamaKamah The people whose minds are filled with Vasanas or worldly tendencies.
Word by word
ते they तम् that भुक्त्वा having enjoyed स्वर्गलोकम् heavenworld विशालम् vast क्षीणे at the exhaustion of पुण्ये merit मर्त्यलोकम् the world of mortals विशन्ति enter एवम् thus त्रयीधर्मम् of the three Vedas अनुप्रपन्नाः abiding by गतागतम् the state of goind and returning कामकामाः desiring desires लभन्ते attain.