In plain words
Having enjoyed that vast heavenly world, when their merit is exhausted they enter the mortal world again. Thus those who follow trayī-dharma and desire pleasures obtain coming and going.
Word for word
They, having enjoyed that vast world of heaven, when merit is exhausted enter the world of mortals; thus following the dharma of the three Vedas, the desirers of desires obtain going and coming.
The commentators
Bhagavadgītābhāṣyam· bhāṣya
ते तं भुक्त्वा स्वर्गलोकं विशालं विस्तीर्णं क्षीणे पुण्ये मर्त्यलोकं विशन्ति आविशन्ति । एवं यथोक्तेन प्रकारेण त्रयीधर्मं केवलं वैदिकं कर्म अनुप्रपन्नाः गतागतं गतं च आगतं च गतागतं गमनागमनं कामकामाः कामान् कामयन्ते इति कामकामाः लभन्ते गतागतमेव, न तु स्वातन्त्र्यं क्वचित् लभन्ते इत्यर्थः ॥ २१ ॥
In plain words
But once they've enjoyed that vast heavenly world, and their merit finally runs out, they fall right back into the world of the mortals. This is the whole shape of it: bound to Vedic ritual alone, nothing more, they only ever get the coming and going — round trip after round trip. Call them what they are: desirers of desires, wanting one thing after another — and all that wanting ever earns them is more coming and going. What they never get, not once, is real freedom.
Close to the Sanskrit
They, having enjoyed that vast, [that is,] extensive, heavenly world, when merit is exhausted, enter, [that is,] return into, the world of mortals. Thus, in the manner stated, [these ritualists,] having resorted exclusively to Vedic ritual action, [that is,] the discipline of the three Vedas, [obtain only] the going and coming — going and coming, [that is,] the departing and returning; desirers of desires, [that is,] 'they desire desires' — thus [they are called] desirers of desires — obtain only this going and coming; but they do not obtain independence in any way — such is the meaning.