In plain words
You alone know Yourself by Yourself, Purushottama—source of beings, Lord of beings, God of gods, Lord of the universe.
Word for word
You Yourself alone know Yourself by Yourself, O Supreme Puruṣa, O source of beings, O Lord of beings, O God of gods, O Lord of the world.
The commentators
Bhagavadgītābhāṣyam
यतः त्वं देवादीनाम् आदिः, अतः —
स्वयमेव आत्मना आत्मानं वेत्थ जानासि त्वं निरतिशयज्ञानैश्वर्यबलादिशक्तिमन्तम् ईश्वरं पुरुषोत्तम । भूतानि भावयतीति भूतभावनः, हे भूतभावन । भूतेश भूतानाम् ईशितः । हे देवदेव जगत्पते ॥ १५ ॥
In plain words
Since you are the origin of the gods and of everything after them — therefore:
You alone truly know your own Self — you, the Lord possessed of unsurpassed knowledge, sovereignty, strength, and every power like these, O Supreme Person. They call you 'nurturer of beings' because you are the very one who brings every being into existence and sustains it, O nurturer of beings. You rule over all beings, O lord of beings. O God of gods, O lord of the whole world.
Close to the Sanskrit
Since you are the origin of the gods and the rest, therefore —
You yourself, by yourself, know the Self, you [know it], possessed of unsurpassed power consisting of knowledge, lordship, strength and the like, the Lord, O Supreme Person. 'One who causes beings to be' — that is, 'the nurturer of beings' — O nurturer of beings. O lord of beings — of beings — the ruler. O god of gods, O lord of the world.