In plain words
You are the Supreme Imperishable to be known; You are the highest support of this universe. You are the changeless guardian of the eternal dharma; I hold You to be the everlasting Puruṣa.
Word for word
You [are] the supreme imperishable, to be known; You [are] the highest resting place of this universe; You [are] undecaying, the guardian of the eternal dharma; the ancient Person You [are], [so] held by me.
The commentators
Bhagavadgītābhāṣyam
इत एव ते योगशक्तिदर्शनात् अनुमिनोमि —
त्वम् अक्षरं न क्षरतीति, परमं ब्रह्म वेदितव्यं ज्ञातव्यं मुमुक्षुभिः । त्वम् अस्य विश्वस्य समस्तस्य जगतः परं प्रकृष्टं निधानं निधीयते अस्मिन्निति निधानं परः आश्रयः इत्यर्थः । किञ्च, त्वम् अव्ययः न तव व्ययो विद्यते इति अव्ययः, शाश्वतधर्मगोप्ता शश्वद्भवः शाश्वतः नित्यः धर्मः तस्य गोप्ता शाश्वतधर्मगोप्ता । सनातनः चिरन्तनः त्वं पुरुषः परमः मतः अभिप्रेतः मे मम ॥ १८ ॥
In plain words
Just from seeing this — this display of your yogic power alone — here is what I now understand —
You are the Imperishable — nothing about you ever decays, and that is exactly why you carry the name. You are Brahman itself, the supreme reality that only those who long for freedom can truly come to know. You are the ultimate resting-place of this whole universe — everything that exists is deposited in you, and there is no refuge beyond you. You never diminish, not by the smallest measure. You are the guardian of the law that has always been and will always be. You are ancient beyond reckoning, and you are the supreme Person — this, at least, is what I have come to know for myself.
Close to the Sanskrit
From this alone, from the sight of your power of yoga, I infer —
You are the Imperishable — it does not perish, thus so called — the supreme Brahman, to be known, to be understood, by those who desire liberation. You are the supreme, excellent repository of this entire universe, of this world — everything is deposited in this, thus 'repository' — the supreme refuge: such is the meaning. Moreover, you are the Undiminishing — your decay does not exist, thus so called — the guardian of eternal law: eternal means ever-existing, permanent; dharma, of that, the guardian — guardian of eternal dharma. You are the ancient, the age-old, the supreme Person — such is held, such is intended, by me.