Since the executive chef of a restaurant can't cook every dish and can't be at every shift and cannot manage every moment that's happening in a larger kitchen, they have sous chefs. A sous chef is like a second-in-command. They're the managers of a kitchen operation. They report to the executive chef, and what they do is manage the more day-to-day operation of the kitchen. Usually sous chefs have responsibility for ordering product, for doing payroll, for hiring and directing staff, and a sous chef is often a step towards increased management like executive sous chef or chef de cuisine or ultimately executive chef.
To become a sous chef, you have to be a great cook. You have to be able to cook and you have to know the technical aspects of cooking, and usually it's the best cooks that have the opportunity to be promoted to sous chef. Now it's not just about cooking skill. It's also about people skills, because ultimately management is getting people to do the work that needs to be done in a good way. If you're somebody who just can't interact with people, even if you're amazing at knife skills or cooking or creating sauces, you're not going to get to be a sous chef, because being a sous chef requires management capability and leadership capability.
To become a great cook, you're going to have to be someone who learns a great deal about food, someone who learns a great deal about the restaurant business, learns about the financial parts of running a kitchen, so that as you develop your cooking skill and you learn more and more about management and business operations, you'll be ready to be promoted to a sous chef position. As you're working as a cook and gaining this experience, you're going to need to prove that you can do all the different aspects of the cooking in that particular kitchen.
You will need to be good at grilling, at sauté, at creating sauces, at garde manger, at prep, because you will ultimately be teaching all of the cooks their positions. And God forbid someone hurts themselves or is out sick, it's very likely a sous chef is going to have to fill in for the night and make it happen.