And the Lord sent fiery serpents... — Hebrew, the serpents, the seraphim (i.e., the burning ones). (See Deuteronomy 8:15; Isaiah 14:29; Isaiah 30:6.) The word appears to denote a particular kind of serpent, as in the following verse. Some think that they were so called because of the bright fiery red upon their heads; others because of the blazing sunbeams on their scales; and others because of their inflammatory and poisonous bite. Venomous snakes are said to abound still in the Arabah.
Related Commentaries of Numbers 21:6
Numbers 21:6
6 And the LORD sent fiery serpents among the people, and they bit the people; and much people of Israel died.