A pitch is an overview of your story.
There are three type of pitches.
The first one is 7 words or less. Within this pitch you should state the genre and the period of time in which the book is set.
The second pitch is 25 words or less. Here you have a little longer to describe your concept and say what the story is about.
The third pitch is 70- 150 words long. This needs to be a proper overview of the whole story that is meant to entice the publisher to want to read it. Even though you have a lot more room to write it is better to write a well written short overview rather than a long overview that rambles on.
All of these pitches need to be strong and to the point. There is no space to ramble on as you want to hook the publisher with not only your idea but also your ability to write an engaging pitch.