Fire in Scripture is consistently the sign of God's holy presence — the burning bush, the pillar of fire, Sinai in flames. Saraph captures the consuming nature of that fire. The burning of the golden calf (Exodus 32:20), the burning of Achan's stolen goods, the burning of offerings — all use saraph. Isaiah's seraphim are the "burning ones" who attend the throne, their very nature reflecting the purifying fire of God's holiness. Saraph also appears in the bronze serpent (Numbers 21:6-9) — the "fiery serpents" sent as judgment, and lifted up in healing, previewing Christ's crucifixion (John 3:14).
Saraph means to burn completely, to consume by fire. It is closely related to the noun seraphim (burning ones, Isaiah 6:2) and describes both destructive fire and purifying fire. Used extensively of judgment, purification, and sacrifice.