Recipes are items that can be found in the chests found in any randomly generated structure such as dungeons and mineshafts. On the tooltip for the recipe it will have 3 random Keyblades these are the recipes that you will learn from it. Mobs also have a 1 in 100 chance of dropping a recipe. These recipes are needed for Synthesis.
To learn the recipes hold it in your hand and right click. If you already have learnt any of the recipes only the ones that you haven’t learnt will be learnt.