Ovens bake, roast and broil foods by heating up the interior cavity using either gas flames or an electric heating element. If you open the oven door during the cooking process, that heat can leak out and result in decreased oven temperatures and longer cook times.
However, you can still check in on your recipes while they cook without opening the oven door. Simply turn on the oven’s interior light and peer through the window to monitor the baking process.