The proscenium is the portion of the stage projecting into the orchestra and lighting pit, in front of the audience. Before, during or after the action, speeches are made on the proscenium, that is the part of the stage jetting out of the curtain edge.
The stage during the time of Shakespeare had no curtain, and projected into the auditorium. So it had no proscenium as such.
Before the play began the director stood in front of the curtain on the proscenium to give a brief introduction to the play.