In plain words
Nourished by yajña, the devas will grant you desired enjoyments. One who enjoys what they give without offering back to them is indeed a thief.
Word for word
For the devas, nourished by yajña, will give to you desired enjoyments; he who enjoys [what is] given by them without offering to them is indeed a thief.
The commentators
Bhagavadgītābhāṣyam
किञ्च—
इष्टान् अभिप्रेतान् भोगान् हि वः युष्मभ्यं देवाः दास्यन्ते वितरिष्यन्ति स्त्रीपशुपुत्रादीन् यज्ञभाविताः यज्ञैः वर्धिताः तोषिताः इत्यर्थः । तैः देवैः दत्तान् भोगान् अप्रदाय अदत्त्वा, आनृण्यमकृत्वा इत्यर्थः, एभ्यः देवेभ्यः, यः भुङ्क्ते स्वदेहेन्द्रियाण्येव तर्पयति स्तेन एव तस्कर एव सः देवादिस्वापहारी ॥ १२ ॥
In plain words
And further—
The gods, nourished and pleased by your sacrifices — that is what 'nourished by sacrifice' means — will give you, in return, exactly the enjoyments you wish for: wives, cattle, children, and everything else you long for. But whoever takes what the gods have given, and enjoys it without giving anything back — without discharging that debt to these very gods — whoever simply gratifies his own body and senses that way, he is nothing less than a thief, nothing less than a robber. He is, in fact, a stealer of what belongs to the gods and to others.
Close to the Sanskrit
And what more—
The desired, intended enjoyments — indeed, to you, to yourselves — the gods will give, will bestow — [i.e.] wives, cattle, offspring, and so on — [the gods being] 'nourished by sacrifice' — [i.e.] increased, pleased, by sacrifices. Such is the meaning. The enjoyments given by those gods — without giving [in return], without having given — [i.e.] without discharging [one's] debt — such is the meaning — to these gods — whoever enjoys [them], [i.e.] merely gratifies his own body and senses, he [is] indeed a thief, indeed a robber; he [is] a stealer of what belongs to the gods and others.