In plain words
All the use there is in a well is contained in a flood that spreads on every side; so all the use there is in all the Vedas is contained in the knowledge of a brāhmaṇa who truly knows.
Word for word
As much use as [there is] in a well, [so much is contained] in water flooded on all sides; so much [as there is] in all the Vedas [is contained in the knowledge] of a brāhmaṇa who knows.
The commentators
Bhagavadgītābhāṣyam
सर्वेषु वेदोक्तेषु कर्मसु यान्युक्तान्यनन्तानि फलानि तानि नापेक्ष्यन्ते चेत् , किमर्थं तानि ईश्वरायेत्यनुष्ठीयन्ते इत्युच्यते ; शृणु —
यथा लोके कूपतडागाद्यनेकस्मिन् उदपाने परिच्छिन्नोदके यावान् यावत्परिमाणः स्नानपानादिः अर्थः फलं प्रयोजनं स सर्वः अर्थः सर्वतः सम्प्लुतोदकेऽपि यः अर्थः तावानेव सम्पद्यते, तत्र अन्तर्भवतीत्यर्थः । एवं तावान् तावत्परिमाण एव सम्पद्यते सर्वेषु वेदेषु वेदोक्तेषु कर्मसु यः अर्थः यत्कर्मफलं सः अर्थः ब्राह्मणस्य संन्यासिनः परमार्थतत्त्वं विजानतः यः अर्थः यत् विज्ञानफलं सर्वतःसम्प्लुतोदकस्थानीयं तस्मिन् तावानेव सम्पद्यते तत्रैवान्तर्भवतीत्यर्थः । ‘यथा कृताय विजितायाधरेयाः संयन्त्येवमेनं सर्वं तदभिसमेति यत् किञ्चित् प्रजाः साधु कुर्वन्ति यस्तद्वेद यत्स वेद’(छा. उ. ४ । १ । ४) इति श्रुतेः । ‘सर्वं कर्माखिलम्’(भ. गी. ४ । ३३) इति च वक्ष्यति । तस्मात् प्राक् ज्ञाननिष्ठाधिकारप्राप्तेः कर्मण्यधिकृतेन कूपतडागाद्यर्थस्थानीयमपि कर्म कर्तव्यम् ॥ ४६ ॥
In plain words
If you are not looking to any of the endless fruits the Veda declares for all these actions, then why go on performing them at all, offering them up "to the Lord"? That is the question raised. Listen —
Think of an ordinary well, or a pond — any small reservoir with only so much water in it. Whatever you can do with that water — bathe, drink, whatever the use — that whole benefit is exactly the same, no more, even if you had an ocean flooding in from every side instead; the smaller benefit is simply absorbed into the larger water, nothing extra gained. It works the very same way with the Vedas. Whatever benefit lies in all the Vedas, in every action they prescribe — the fruit those actions yield — that very benefit, for the brāhmaṇa who has renounced the world and come to know the ultimate truth, whatever is the fruit of that knowledge, which stands in the place of the boundless flood-water, that same amount is already there, simply contained within it, nothing more to add. As the Upaniṣad says: 'Just as lesser throws of the dice flock to the one who has won with the highest throw, so everything good that people do comes, in the end, to the one who knows this' (Chāndogya Upaniṣad 4.1.4) — this is what scripture itself tells us. And Kṛṣṇa himself will later say, 'all action, entirely' (Bhagavad Gītā 4.33), making the same point. So, before someone has become eligible for the path of knowledge, while they are still qualified only for action, they should still perform that action — even though what it offers is only ever the well-and-pond scale of benefit.
Close to the Sanskrit
If those endless fruits which are declared for all the actions stated in the Veda are not looked to, then for what purpose are those actions performed with the thought 'for the Lord'? — thus it is said. Hear —
Just as, in the world, in any one of many [sources] such as a well or pond — a reservoir of limited water — whatever purpose, of whatever measure, such as bathing, drinking, and so on, that entire purpose, even in water flooding on every side, is accomplished to that same extent only — it is simply included in that [greater water]. Such is the meaning. In the same way, that same measure only is accomplished: whatever purpose there is in all the Vedas, in the actions enjoined by the Veda, which is the fruit of action — that purpose, for the brāhmaṇa, the renunciate, who knows the truth of the ultimate reality, whatever purpose, whatever is the fruit of knowledge, standing in the place of water flooding on every side — in that, that same amount only is accomplished; it is simply included there. Such is the meaning. 'As the lesser throws flock to one who has won by the throw called Kṛta, so all that people do rightly, whatever it is, comes to him who knows this, who knows that' (Chāndogya Upaniṣad 4.1.4) — thus, because of the scriptural statement. 'All action, entirely' (Bhagavad Gītā 4.33) — thus he will also say. Therefore, before attaining eligibility for the discipline of knowledge, by one qualified [only] for action, action — even though it stands in the place of the [limited] benefit of a well or pond — is to be performed.