Any component in a recipe can have preparations assigned that provide detail about how that component should be treated in the recipe. Preparations can be used to adjust trim yield, convey prep details, and provide any component-level details a person might need to produce the recipe.
The "Preparations" page is where you can keep track of all the preparations that are applied across different recipes. If you select a preparation, it will show you all the recipes that include that preparation, as well as the specific ingredient or subrecipe that the preparation is linked to. You can click on any row to go directly to the recipe that uses the preparation.
To see how you can use preparations to adjust trim yields, check out this article ("How do I account for trim yields?").