Seraphim appear by that exact name only once in the Hebrew Bible, in Isaiah's inaugural vision in the Temple, chapter six. Isaiah sees the Lord enthroned, attended by seraphim, each with six wings, two covering the face, two covering the feet and two used for flying, calling to one another, holy, holy, holy is the Lord of hosts, the whole earth is full of his glory. One seraph then takes a live coal from the altar with tongs and touches it to Isaiah's mouth, declaring his guilt taken away and his sin atoned, before Isaiah accepts his prophetic commission. The name itself, from a Hebrew root connected to burning, and the fiery imagery of that scene, are essentially the entire biblical basis on which the whole later, much more elaborate doctrine of Seraphim as the highest of nine angelic choirs was eventually built.
Pseudo-Dionysius reads their name as meaning kindling or burning and their ceaseless motion around the divine throne as constancy, warmth and keenness carried to the highest possible pitch, an interpretation that later medieval theologians, including Thomas Aquinas, accepted as authoritative even though it develops Isaiah's brief, physically concrete vision into an abstract description of pure, unwavering divine love that the prophet's own six verses never actually state.