A directorial concept or director's vision is the creative element that the director contributes to the production. It describes their interpretation of the play and how the want it to be presented. A clear vision can:
gives the play a clear purpose or intention, a unified look, mood or atmosphere.
create a unifying focus which can be communicated not only through dialogue, but also the elements of productin.
helps actors focus on the bigger picture outside their individual character.
Visions do not have to be complicated. The simpler the vision, the easier it will be to communicate to you designers, actors and crew.
For tips on how to create a directorial concept read the blog Creating the Director's Vision.