In plain words
There the son of Pandu then saw the whole universe, divided in countless ways yet gathered in one, within the body of the God of gods.
Word for word
There the son of Pāṇḍu then saw the whole world, standing in one [place] and divided in many ways, in the body of the God of gods.
तत्रैकस्थं जगत् कृत्स्नं प्रविभक्तमनेकधाअपश्यद् देवदेवस्य शरीरे पाण्डवस्तदा
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
किञ्च —
तत्र तस्मिन् विश्वरूपे एकस्मिन् स्थितम् एकस्थं जगत् कृत्स्नं प्रविभक्तम् अनेकधा देवपितृमनुष्यादिभेदैः अपश्यत् दृष्टवान् देवदेवस्य हरेः शरीरे पाण्डवः अर्जुनः तदा ॥ १३ ॥
In plain words
And there is more —
'There' means: in that universal form. 'Abiding in one' means: resting in that single form. And what rested there was the whole world, 'divided many times over' — that is, split into gods, ancestors, human beings and every other kind. 'He saw' — he actually beheld it, there in the body of Hari, the God of gods. 'The son of Pāṇḍu' is Arjuna. And 'then' is that very moment.
Close to the Sanskrit
And what more —
'There' — that is, in that universal form; 'abiding in one' — that is, situated in one; the whole world, 'divided manifoldly' — that is, by the distinctions of gods, ancestors, humans and the rest; 'he saw' — he beheld — in the body of Hari, the God of gods; 'the son of Pāṇḍu' — Arjuna; then.