Your main dishes would be considered part of dinner or supper time meals in the evenings. Cooking that roast, seafood, or pasta would be considered your main dish because it is the focus of the evening meal. Everything else that accompanies it, whether it be salad or vegetable, likely represents your side dish.
You can save time and money well ahead of the meal time by planning your main dishes out a week in advance. This would of course depend on how often you are grocery shopping. When you make that trip to the store, if you can plan everything out in advance for your main dishes you will know what you need and what you have to pick up in advance for them.
The main dishes in your meals are what will make it stand out. If you plan to entertain a variety of guests for a dinner, let them know in advance what your main course will be. If you know your guests well enough, you might be able to tailor your main dish based on something everyone will like, thus helping you avoid any potential conflict with a guest's taste.
Whatever your pleasure for a main dish, the possibilities are endless.