They are the angels who are in communication with God. They purify and illuminate.

The Seraphim surround the Throne of God and belong to the highest Order of the angelic hierarchy, being the only ones to sing the Music of Heaven.
Belonging to the Highest Hierarchy or Sphere (Upper Triad) and are on the limit between the created and the uncreated. They serve as divine advisors. 
They are on the 10th dimension of consciousness