In plain words
Filled with desire and intent on heaven, they prescribe many elaborate rites that bring rebirth as their fruit and aim at enjoyment and power.
Word for word
...desire-natured, intent on heaven, [proclaiming speech] that gives birth as the fruit of action, abounding in special rites, aimed at the attainment of enjoyment and lordship...
The commentators
Bhagavadgītābhāṣyam
ते च —
कामात्मानः कामस्वभावाः, कामपरा इत्यर्थः । स्वर्गपराः स्वर्गः परः पुरुषार्थः येषां ते स्वर्गपराः स्वर्गप्रधानाः । जन्मकर्मफलप्रदां कर्मणः फलं कर्मफलं जन्मैव कर्मफलं जन्मकर्मफलं तत् प्रददातीति जन्मकर्मफलप्रदा, तां वाचम् । प्रवदन्ति इत्यनुषज्यते । क्रियाविशेषबहुलां क्रियाणां विशेषाः क्रियाविशेषाः ते बहुला यस्यां वाचि तां स्वर्गपशुपुत्राद्यर्थाः यया वाचा बाहुल्येन प्रकाश्यन्ते । भोगैश्वर्यगतिं प्रति भोगश्च ऐश्वर्यं च भोगैश्वर्ये, तयोर्गतिः प्राप्तिः भोगैश्वर्यगतिः, तां प्रति साधनभूताः ये क्रियाविशेषाः तद्बहुलां तां वाचं प्रवदन्तः मूढाः संसारे परिवर्तन्ते इत्यभिप्रायः ॥ ४३ ॥
In plain words
And they —
'Whose very self is desire' simply means: their nature is desire, desire is what they are devoted to. 'Devoted to heaven' means: for these people heaven is the highest goal a human being can reach — that's exactly why they're called devoted to heaven, heaven above everything else. The speech that 'bestows birth as the fruit of action' means just this: the fruit of action, here, is birth itself — and that speech is what hands out birth as that fruit. And 'they proclaim' carries straight over from the line before. 'Abounding in specific rites' means: particular ritual acts, many of them, are exactly what fills this speech — the speech by which goals like heaven, cattle, and children get proclaimed again and again, lavishly. 'The attainment of enjoyment and lordship' means: enjoyment and lordship together, and reaching, acquiring, those two — that is what's meant. And it is toward that goal that these specific rites function as means; and caught up in a speech thick with exactly such rites, proclaiming it endlessly, the deluded simply go on turning, around and around, in saṃsāra. That is the sense of it.
Close to the Sanskrit
And they —
'Whose self is desire' [means] whose nature is desire, [i.e.] devoted to desire — such is the meaning. 'Devoted to heaven' [means]: heaven [is] the highest human aim for whom — they [are] 'devoted to heaven', for whom heaven is foremost. 'That which bestows birth as the fruit of action' [means]: the fruit of action [is] birth itself — the fruit of action — birth as the fruit of action — that it gives — thus [it is called] 'the bestower of birth as the fruit of action' — that speech. 'They proclaim' is to be carried over [from the previous verse]. 'Abounding in specific rites' [means]: the particular kinds of actions [are] specific rites; those, abundant, in which speech [they occur] — that [speech], by which the aims such as heaven, cattle, offspring and so on are abundantly proclaimed. 'The attainment of enjoyment and lordship' [means]: enjoyment and lordship [together] — the attainment, the acquisition, of these two [is] 'the attainment of enjoyment and lordship' — with a view to that, the specific rites which serve as means, [that speech] abounding in those, proclaiming that speech, the deluded revolve in saṃsāra. Such is the purport.