Technique: Preheat Oven
Before baking or roasting something in an oven, you usually want to bring it up to temperature in advance, this is known as preheating. There are a few reasons for this, the first is that the timing on most recipes is calculated assuming you're starting to bake at a higher temperature. If it takes 10-15 minutes for your oven to get hot, you will need to add this time to your total cooking time.
Food may also not cook properly if you do not preheat. Some recipes rely on a high heat at the beginning to brown food and then lower the temperature later. Delicate baked goods, like some cakes and pastries, need a consistent temperature for maximum rising. If they are put in an oven that isn't hot enough, they will be heavier and not rise as well.
You lose heat every time you open the oven door. To compensate for this loss, some recipes will have you preheat the oven to a slightly higher temperature and then immediately reduce it once the food is in the oven.