What is "ur-hamlet"

The Ur-Hamlet (the German prefix Ur- means "primordial") is the name given to a play by an unknown author, thought to be either Thomas Kyd, an English dramatist or the more well known English playwright, William Shakespeare. Dated back by scholars to the second half of 1587, no surviving copy of the play remains today. The play was known to be played in London as a stage play, more specifically at The Burbages Shoreditch Playhouse as recalled by Elizabethan author Thomas Lodge. The play is known to have a character in the play named Hamlet. The only other known fact about the play is that there is a ghost in the play and he cries "Hamlet, revenge!".