The seal was a symbol of power, utilised by Rassilon during the war against the Great Vampires and Old Ones.
The Seal was used as a symbol of identification, and was viewed with deep reverence. Time Lords displayed the Seal of Rassilon in places of great power. The Seal was said to be an ancient symbol for warding off evil.
When he sent the Master into the Death Zone to rescue the Doctor, Lord President Borusa gave the Master the “Seal of the High Council,” a round metallic object emblazoned with the Seal of Rassilon, hoping it would convince the Doctor of the Master’s good intentions.
At some point, the Time Lords also used a different seal, bearing Gallifreyan markings.