Simple answer:
It is a list of preventions and alternatives to expected and unexpected problems.

The simplest way to do this is to make a table with three columns. In the first column you list everything that could go wrong with your project. In the second column you show for each problem the steps you are taking to prevent that from happening. In the third column you show what you do when it happens.

This simple device is a great way to show people (customers, vendors, investors, all stakeholders, etc.) that you are on top of the situation, plan ahead, and ultimately, are the right person to make their product. Surprisingly, very few people do this. Instead, they react to problems. Reactions often waste time and money and go down the wrong path.

Be proactive and come up with your own risk and contingency plan.