In plain words
Salutations before You and behind You; salutations to You on every side, O All. Infinite in power and immeasurable in might, You pervade everything; therefore You are all.
Word for word
Salutation to You in front and behind; salutation be to You on every side, O All; You, of endless power and measureless valour, pervade all; therefore You are all.
The commentators
Bhagavadgītābhāṣyam
तथा —
नमः पुरस्तात् पूर्वस्यां दिशि तुभ्यम् , अथ पृष्ठतः ते पृष्ठतः अपि च ते नमोऽस्तु, ते सर्वत एव सर्वासु दिक्षु सर्वत्र स्थिताय हे सर्व । अनन्तवीर्यामितविक्रमः अनन्तं वीर्यम् अस्य, अमितः विक्रमः अस्य । वीर्यं सामर्थ्यं विक्रमः पराक्रमः । वीर्यवानपि कश्चित् शत्रुवधादिविषये न पराक्रमते, मन्दपराक्रमो वा । त्वं तु अनन्तवीर्यः अमितविक्रमश्च इति अनन्तवीर्यामितविक्रमः । सर्वं समस्तं जगत् समाप्तोषि सम्यक् एकेन आत्मना व्याप्नोषि यतः, ततः तस्मात् असि भवसि सर्वः त्वम् , त्वया विनाभूतं न किञ्चित् अस्ति इति अभिप्रायः ॥ ४० ॥
In plain words
And in the same way —
Salutation to you from the east, from in front; salutation to you from behind, and from behind again; salutation to you from every side, in every direction, everywhere you stand — O All-in-one. Your 'infinite valor, immeasurable prowess': the strength inside you has no limit, and neither does what you actually do with it. Strength is the capacity within; prowess is that capacity spent in action — and Śaṅkara points out that plenty of strong people never quite act, or act half-heartedly, when courage is called for. Not you. Your strength is boundless, and so is what you do with it. You fill this entire world, completely, with the one Self that you are — and because that is so, you are everything. Apart from you, nothing at all exists, not even the smallest thing. That is the point.
Close to the Sanskrit
Likewise —
Salutation from in front, in the eastern direction, to you; then from behind to you, from behind too, salutation be to you; salutation to you from everywhere indeed, in all directions, everywhere, to you who stand there — O All. [The compound] 'of infinite valor and immeasurable prowess' [means]: infinite is the valor of this one, immeasurable is the prowess of this one. Valor means capability; prowess means exertion-in-action. Even one possessing capability sometimes does not exert himself in a matter such as slaying an enemy, or is of feeble prowess. But you are of infinite capability and immeasurable prowess — thus [you are] 'of infinite valor and immeasurable prowess'. You completely pervade the entire whole world, properly, by one Self; because [this is so], therefore you are, you become, all — you are everything; apart from you, not the slightest thing exists — this is the purport.