The Concept is the single idea or message or thought around which your piece is built. It’s the thing that holds your work together, harmonizing all its different elements into a single clear voice. Without it, there is only cacophony.
The notion of having a well-defined concept to your work may seem like a no-brainer, but it is actually a very slippery thing to handle. Here’s a sad example: in one feature film I worked on, I had lunch with the director over which it became painfully clear he had no idea what the film was actually about. This was about a year into the production. we’re not talking about some inexperienced student, mind you: the director was a hyper-talented person with 40 years experience in film-making, working with the best and brightest in the industry. Still he was unable to create and maintain a clear, consistent concept. The film, by the way, was canceled about a month after that lunch took place. I wasn’t at all surprised. I’ve seen blurry concepts kill promising projects before.