In plain words
Among the Adityas I am Vishnu; among lights, the radiant sun; among the Maruts I am Marichi; among the stars I am the moon.
Word for word
Among the Ādityas I [am] Viṣṇu; among lights, the radiant sun; Marīci among the Maruts I am; among the stars I [am] the moon.
The commentators
Bhagavadgītābhāṣyam
एवं च ध्येयोऽहम् —
आदित्यानां द्वादशानां विष्णुः नाम आदित्यः अहम् । ज्योतिषां रविः प्रकाशयितॄणाम् अंशुमान् रश्मिमान् । मरीचिः नाम मरुतां मरुद्देवताभेदानाम् अस्मि । नक्षत्राणाम् अहं शशी चन्द्रमाः ॥ २१ ॥
In plain words
And this is how I am to be held in meditation —
Of the twelve Ādityas, I am the one called Viṣṇu. Among the lights — that is, among the bodies that do the illuminating — I am the sun, aṃśumān, the one made of rays. Among the Maruts, the storm-gods in all their ranks, I am the one called Marīci. Among the stars I am śaśī, the moon.
Close to the Sanskrit
And thus I am the one to be meditated upon —
Of the Ādityas, of the twelve, the Āditya named Viṣṇu am I. Of lights — of illuminators — the sun, aṃśumān, that is, ray-bearing. Of the Maruts — of the divisions of Marut deities — I am the one named Marīci. Of the stars I am śaśī, that is, the moon.