The concept is quite an important step in the creative process. Some people like to do it in writing to maybe share it with others, some just do it for themselves. Like the preceding paragraph stated, it's important to be able to explain your decisions. So at first, you need to know the purpose of your 3D-model. Sometimes, you may get such a concept from your Art Director, or you're given the freedom to create your own. To illustrate the latter case, let's imagine a character and do this process in short, so you can see how much this first concept can improve the process and the resulting piece of work.
This example will be written like a really short background story for the character, to give it some personality. Again, there are many ways of doing this and you are encouraged to find the ones that work best for you, because it depends on your personal style of working.