The mouth plays a critical role in digestion. Proper chewing and a complete bite are crucial in this process.
Imagine ... you are enjoying a meal. You start by taking a bite of food. Yum! The flavors ignite the salivary glands. The saliva starts flowing. You begin enjoying all the delicious flavors. The teeth are chewing in order to break down the food into smaller and smaller bits. The tongue moves the food around the mouth to make sure it is getting properly tasted and broken down. The saliva encases the food and its enzymes break it down further. By the time you are ready to swallow the bolus of food is nicely mashed and easily goes down the throat toward the stomach. This happens each time you take a bite of food.
Now imagine that you are missing several (or even a few) teeth. You take a bite of food but it is harder to grind that food up. When you swallow there are larger chunks of food present in the bolus that is heading toward the stomach. The stomach has to work harder to break down this mixture. You may notice stomach discomfort after eating or an increase in acid reflux. This also happens when you don't properly chew the food (like inhaling it instead of enjoying it).
If you are missing teeth contact your dental professional to see what options you have to correct this. Also make sure to chew your food completely to help with the digestive process.