te tam bhuktvaa svargalokam vishaalam ksheene punye martyalokam vishanti |
evam trayeedharmamanuprapannaa gataagaam kaamakaamaa labhante || 21 ||

Having enjoyed the (joys of) mighty heaven, they, with their merits exhausted, return to the mortal world. In this manner, those with selfish desires, who take refuge in the three-fold system, arrive and depart (repeatedly).
te : they
tam : that
bhuktvaa : enjoyed
svargalokam : heaven
vishaalam : mighty
ksheene : exhausted
punye : merits
martyalokam : mortal world
vishanti : come to
evam : in this manner
trayeedharmam : three-fold system
anuprapannaahaa : take refuge
gataagaam : arrival and departure
kaamakaamaahaa : those with selfish desires
labhante : attain
Shri Krishna continues the description of desire-oriented devotees (sakaam bhaktas) who worship Ishvara with a view to gain earthly and heavenly rewards and conforts. Shri Krishna says that their efforts will bear fruit on earth as well as in heaven. However, he says that their stay in heaven is temporary. After their stay ends, they once again come back to earth and get caught up in the endless cycle of birth and death.
As an example, consider the plight of those people who work hard, earn a good living but have trouble managing their spending. As soon as they get their salary, they instantly spend most of it on the first of the month, and barely manage to pay their bills for the remainder of the month. They somehow forget that fact that money, like anything else in this world, is finite.
Similarly, people who enjoy heavenly pleasures stay in heaven until their merits (punya) are depleted. Once that happens, they come back to earth and are born into a human form if they are lucky, or some other form if they are not.
Shri Krishna says that such people follow the “three-fold system”. This refers to Vedic knowledge comprising the three modes or gunaas. Since the gunaas themselves are finite and transient, the results attained by the followers of these gunaas will also be finite and transient.
So therefore, although the desire-oriented devotee is better than someone who has no room for devotion, he is still trapped in a pursuit of comfort in pleasure in this world as well as in other worlds. Unlike such a devotee, what we should truly desire is liberation or moksha.
What kind of devotee gets liberation? This is taken up next.