The Hebrew word saraph / seraphim appears three times in the Torah (Numbers 21:6, 21:8, Deuteronomy 8:15) and four times in the Book of Isaiah (6:2, 6:6, 14:29, 30:6). In most of these passages the word simply refers to “fiery” snakes, “fiery” possibly referring to the burning effect of poison. But in Isaiah 6:2 and 6:6 the term is used to describe “Fiery Ones” with 6 wings that attend to the throne of God and proclaim his holiness. It is clear from their proximity to the throne that these heavenly attendants are very powerful.
Seraphim also appear in the intertestamental Book of Enoch where they are mentioned, in conjunction with cherubim, as the heavenly creatures standing nearest to the throne of God. In the Second Book of Enoch, two classes of celestial beings are equated with the seraphim and cherubim, known as the phoenixes and the chalkydri (which literally means “brazen hydras” or “copper serpents”). Both are described as “flying elements of the sun” that reside in either the fourth or sixth heaven, who have twelve wings and burst into song at sunrise.
In the Book of Revelation there are references to “four living creatures” that surround the throne on God, however the account differs slightly from the account of Isaiah so it is unclear whether these are the same beings or different ones.
There is considerable speculation in later Christian and Jewish mystical and magical texts as to how powerful the seraphim actually are. In the Celestial Hierarchy, Pseudo-Dionysius identifies them as the highest of the nine orders of angels. In the Zohar, they are ranked third. Others rank them differently again. Personally I am inclined to agree with Pseudo-Dionysius but I sometimes wonder if it’s better left a mystery.