This fancy French term means that you have all of the ingredients, utensils, cooking pots and pans, baking pans, and measuring cups and spoons on hand and on the counters before you start to cook. For some people, it just means that you have everything on hand before you start cooking. For others, it means that every ingredient is chopped, peeled, minced, or grated before cooking is started. This is especially important in stir-frying, when the cooking process is so fast that the recipe would be ruined if the cook had to stop to search for an ingredient.
Mise en place has more importance in restaurant kitchens, where there are some chefs, known as sous chefs, whose sole job is to prepare ingredients for the chef to cook with.
Why is It Important?
First, it's always wise to make sure you have all of the ingredients needed in a recipe before you start cooking. Read through the recipe a couple of times and gather all of the ingredients. If you want to make substitutions or use equivalents for some of the foods called for in the recipe, make sure that you have the right amount of those foods too.
Second, once you start cooking, it interrupts the flow if you have to stop to search for something. It's easy to make a mistake and omit an ingredient or overlook a preparation step if you go to another room or check the pantry or your kitchen drawers for some equipment, spice, or ingredient.
And finally, cooking is just more pleasant when everything is in its place. You can enjoy the rhythm of cooking and baking. Listen to how the food sounds as it's being prepared. The "thunk" of a knife cutting through mushrooms, how meat sounds and smells and the sizzle when it hits a hot pan, and the rhythmic pounding of a stand mixture should be enjoyed.
Try developing your own plan for mise en place. You don't have to have everything prepared down to the last bit of grated cheese before you start cooking, but find your own rhythm in the kitchen. You will find it is easier to cook, you'll be able to cook more efficiently and quickly, and you will enjoy the whole process more.