Anh hùng gì? Anh hùng rơm, Tôi cho nắm lửa hết cơn anh hùng.

What hero? Just a straw hero, I will let him hold fire for this hero fever to pass.

This is equivalent to “barking dogs never bite” (i.e., someone who talks big but is a coward). A bundle of straw burns intensely but quickly disintegrates.