Like any industry, the kitchen has its own lingo. Many of the terms used in a professional kitchen come from France. One of the first terms we learned in school was mirepoix.
Quite simply, mirepoix refers to a mix of 2 parts onions, 1 part carrots, and 1 part celery. There are variations on mirepoix which include parsnips, bell pepper, or leeks. The size of the dice depends on what kind of recipe you are making. The shorter cooking time of a recipe, the smaller the dice.. Likewise, the longer the cooking time, the larger the dice. This principle can be applied to most recipes!
Onions, carrots, and celery are humble ingredients you can pick up inexpensively at nearly any grocery. Mirepoix helps to form the backbone of a kitchen. Off the top of my head I can think of more recipes that use mirepoix (stocks, soups, sauces, rice pilaf, braises) than ones that don’t.