In plain words
As the many rushing currents of rivers flow toward the ocean alone, so these heroes of the human world enter Your blazing mouths.
Word for word
As the many rushing waters of the rivers run, facing it, to the ocean alone, so these heroes of the world of men enter Your mouths, blazing all round.
The commentators
Bhagavadgītābhāṣyam
कथं प्रविशन्ति मुखानि इत्याह —
यथा नदीनां स्रवन्तीनां बहवः अनेके अम्बूनां वेगाः अम्बुवेगाः त्वराविशेषाः समुद्रमेव अभिमुखाः प्रतिमुखाः द्रवन्ति प्रविशन्ति, तथा तद्वत् तव अमी भीष्मादयः नरलोकवीराः मनुष्यलोके शूराः विशन्ति वक्त्राणि अभिविज्वलन्ति प्रकाशमानानि ॥ २८ ॥
In plain words
How do they enter the mouths? To this he says:
Think of rivers in full flow: their many currents of water, each one racing at its own speed, all set squarely toward the ocean, running into it and entering it. In just that way these heroes of the human world — Bhīṣma and the rest, warriors among men — are entering your mouths, which are blazing, ablaze with light. || 28 ||
Close to the Sanskrit
"How do they enter the mouths?" — thus, he says:
Just as of rivers that are flowing, the many — that is, numerous — water-currents, i.e. currents of waters, particular hastenings, run — that is, enter — toward the ocean alone, being face-turned toward it, that is, set directly toward it; so, in that same way, these heroes of the world of men of yours, namely Bhīṣma and the rest, warriors in the human world, enter your mouths, which are blazing forth, that is, shining. || 28 ||