A signet ring was a type of ring which featured some form of emblem.
The First Doctor wore a signet ring consistently throughout his life. (TV: An Unearthly Child - The Tenth Planet, Twice Upon a Time) It did not fit the Second Doctor's finger, which Ben Jackson claimed proved that he wasn't the Doctor. (TV: The Power of the Daleks)
The Master also wore a signet ring in his "Harold Saxon" incarnation, which featured an emblem resembling the Lazarus Laboratories logo. (TV: The Sound of Drums) The ring stored genetic information that was later used by a cult to resurrect him. (TV: The End of Time)