Arjuna was describing Krishna cosmic form and while describing he see that all his enemies ( who are his opponents ) in Kauravas are actually eaten up by Krishna .
Everything in this world gets dissolved or eaten up after some time. Arjuna here metamorphically talks about one of the very fundamental understanding of life, that everything which gets created , gets again dissolved .
Arjuna was having a lot of fear about his opponent like Drona ( who is teacher ) , Bhisma ( who is a great warrior and grandfather of Arjuna ) after seeing that they are all getting consumed into Krishna’s mouth throught the teeth, he becomes free from fear. He now start realizing the inevitable truths of the world .