Whitepaper is the basic document for every ICO project.
It's like a standard business plan you would need to prepare for any brick&mortar business, that you need fundign for.
A white paper should contain all the important information about the project, the vision of the authors, the problems it solves ... in other words, the Whitepaper should sell the project and make investors interested.
A poorly prepared whitepaper is a great sign of a scam ICO that you should stay away from.