Ideas often start off as lofty visions of the way something should be or how something should be done. Most people can take an idea and figure out how to make it work in a given situation. They may be able to make a mental model or even a sketch of their concept and determine what might be needed to move an idea forward. The parts or pieces of the concept may even become apparent at this stage.
This is the point where many people will wrap up the design process and call it a day. You've come up with the big idea, you've worked out the major and minor aspects, and you can talk comfortably about what you have in mind. However, you need to resist this urge and go deeper in order to develop the details of your proposal.
The intent is not to fully define what you are doing, but rather to add clarity and to sharpen the edges of your vision. The more that you can describe how your solution works, how it addresses a specific problem, and the impact that it makes on your customer, the better the position you will be in to take your work to the next step in it's development. It might also help you make a sale or convince someone that your idea warrants further attention.
Working out the details will help bring people in to your vision and establish a connection with it. Not everyone will like what they see and may move on and that's ok. The point, however, is to create an emotional reaction in the person viewing your work and to get them excited about what you are doing. Connections are made when people can see not only your big idea, but also how you are going to turn it into reality.