In plain words
Here, joy of the Kurus, resolute buddhi is single-pointed; the buddhis of the irresolute branch endlessly in many directions.
Word for word
Resolute buddhi is one here, O joy of the Kurus; for many-branched and endless are the buddhis of the irresolute.
The commentators
Bhagavadgītābhāṣyam
येयं साङ्ख्ये बुद्धिरुक्ता योगे च, वक्ष्यमाणलक्षणा सा —
व्यवसायात्मिका निश्चयस्वभावा एका एव बुद्धिः इतरविपरीतबुद्धिशाखाभेदस्य बाधिका, सम्यक्प्रमाणजनितत्वात् , इह श्रेयोमार्गे हे कुरुनन्दन । याः पुनः इतरा विपरीतबुद्धयः, यासां शाखाभेदप्रचारवशात् अनन्तः अपारः अनुपरतः संसारो नित्यप्रततो विस्तीर्णो भवति, प्रमाणजनितविवेकबुद्धिनिमित्तवशाच्च उपरतास्वनन्तभेदबुद्धिषु संसारोऽप्युपरमते ता बुद्धयः बहुशाखाः बह्वयः शाखाः यासां ताः बहुशाखाः, बहुभेदा इत्येतत् । प्रतिशाखाभेदेन हि अनन्ताश्च बुद्धयः । केषाम् ? अव्यवसायिनां प्रमाणजनितविवेकबुद्धिरहितानामित्यर्थः ॥ ४१ ॥
In plain words
The understanding set out in Sāṅkhya, and in Yoga as well — the one whose marks are about to be described — that understanding:
There is one understanding — resolved, certain, whole — and it simply overrides every branching, contrary view opposed to it, because it is born of sound, valid knowledge. This is the path of the highest good, Kurunandana. But the other understandings, the contrary ones, keep dividing and sub-dividing, and it is exactly that spreading, branching confusion that makes saṃsāra feel endless, boundless, ceaseless, stretched out forever, vast beyond measure. And it is exactly the opposite — the discriminating understanding born of real knowledge — that brings it all to rest: once the endlessly divided views fall still, saṃsāra itself falls still. 'Many-branched' simply means this: many are the branches, so the understandings that have them are called many-branched, many-divided. That's all it means. And because every single branch differs from every other, the understandings themselves are endless too. Endless for whom? For the irresolute — for those without the discriminating knowledge that only sound understanding can give. That is the sense of it.
Close to the Sanskrit
That understanding which was stated in Sāṅkhya and in Yoga, having the characteristics about to be described — that —
The understanding whose essence is resolve, whose nature is certainty, one alone, overrides the branching divisions of other, contrary understandings, because it is produced by correct means of valid knowledge — here, on the path of the highest good, O Kurunandana. But those other, contrary understandings — because of the influence of the spread of their branching divisions, saṃsāra becomes endless, boundless, unceasing, perpetually extended, vast; and owing also to the influence of the discriminative understanding produced by valid means, when the endlessly-divided understandings have ceased, saṃsāra too comes to rest. Those understandings are 'many-branched' — 'many' are the 'branches' of which, those are 'many-branched', 'many-divided' — this is that [meaning]. For by the distinctness of each branch, endless too are the understandings. Of whom? Of the irresolute — devoid of the discriminative understanding produced by valid means. Such is the meaning.