The "Preparations" page is where you can keep track of all the preparations that get applied to your ingredients in different recipes. It can also be used to track trim yields, cook-offs, and quality controls (e.g., parsley, chopped). You can click into a preparation to see its usages. You can also click into a recipe and see the ingredient that the preparation is attached to.
To see how you can use preparations to adjust trim yields, check out this article ("How do I account for trim yields?").