It can be termed as the planning phase of the project which is focussed on concept development and idea along with the writing of initial design documents. It is actually making the game development team ready prior to the development. The central goal is, however, to complete the game design, freeze on the color concept, style, pipeline etc and come up with good documentation, do good technical prototyping to demonstrate its feasibility. It’s like having a proof of concept. If a game concept has passed the pre-production stage it means that your team is fit enough to make the game and the game is well worth the effort.