In plain words
As moths rush with great speed into a blazing fire to perish, so too these creatures rush with great speed into Your mouths for destruction.
Word for word
As moths enter a blazing fire for destruction, their speed swelling, just so the creatures enter Your mouths too for destruction, their speed swelling.
The commentators
Bhagavadgītābhāṣyam
ते किमर्थं प्रविशन्ति कथं च इत्याह —
यथा प्रदीप्तं ज्वलनम् अग्निं पतङ्गाः पक्षिणः विशन्ति नाशाय विनाशाय समृद्धवेगाः समृद्धः उद्भूतः वेगः गतिः येषां ते समृद्धवेगाः, तथैव नाशाय विशन्ति लोकाः प्राणिनः तवापि वक्त्राणि समृद्धवेगाः ॥ २९ ॥
In plain words
Why are they entering, and in what way? Here is how Arjuna answers —
Just as moths — winged things — fly into a blazing fire, into fire itself, and fly in to be destroyed, to be wiped out, and fly in at full speed: their speed, their onward rush, has surged to its very peak, and that is what 'full-speed' means here — in exactly that way the worlds, the breathing creatures, fly into your mouths as well, to be destroyed, at that same full speed.
Close to the Sanskrit
For what purpose do they enter, and how? — thus he says —
Just as moths — pataṅgāḥ means winged creatures — enter a blazing fire — jvalanam means fire — for destruction, that is, for annihilation, being samṛddhavegāḥ: those of whom the vega, the motion, is samṛddha, that is, risen to its full height, they are samṛddhavegāḥ — in exactly that same way the worlds, that is, living beings, enter your mouths too for destruction, being samṛddhavegāḥ.