In plain words
You are the primal Deity, the ancient Puruṣa; You are the supreme refuge of this universe. You are the knower and the known, and the supreme abode. By You this universe is pervaded, O infinite Form.
Word for word
You [are] the first God, the ancient Person; You [are] the highest resting place of this universe; You are the knower and the knowable and the supreme abode; by You the universe [is] pervaded, O You of endless form.
The commentators
Bhagavadgītābhāṣyam
पुनरपि स्तौति —
त्वम् आदिदेवः, जगतः स्रष्टृत्वात् । पुरुषः, पुरि शयनात् पुराणः चिरन्तनः त्वम् एव अस्य विश्वस्य परं प्रकृष्टं निधानं निधीयते अस्मिन् जगत् सर्वं महाप्रलयादौ इति । किञ्च, वेत्ता असि, वेदिता असि सर्वस्यैव वेद्यजातस्य । यत् च वेद्यं वेदनार्हं तच्च असि परं च धाम परमं पदं वैष्णवम् । त्वया ततं व्याप्तं विश्वं समस्तम् , हे अनन्तरूप अन्तो न विद्यते तव रूपाणाम् ॥ ३८ ॥
In plain words
Once more he takes up its praise —
You are the first of all gods, because you are the one who made this world. You are the Person, because you dwell within this very body of ours — the ancient one, older than time itself. You alone are the ultimate resting-place of this whole universe, the one in whom everything is gathered back at the great dissolution. You are the knower of everything there is to know — and more than that, you are the very thing known. You are the supreme abode, the highest state, Viṣṇu's own dwelling. This entire universe is stretched through and filled by you — and, O one of infinite form, your forms simply never end.
Close to the Sanskrit
Again also he praises —
You are the primal god, because of being the creator of the world. You are the Person, because of dwelling in the body; the ancient, the age-old one; you alone are the supreme, excellent resting-place of this universe — in you the whole world is deposited at the time of the great dissolution, thus. Moreover, you are the knower, you are the one who knows the entire class of knowable things. And whatever is to be known, worthy of being known, that too you are; and you are the supreme abode, the supreme state, belonging to Viṣṇu. By you this entire universe is extended, pervaded; O one of infinite form, there is no end to your forms.