Food combining is a whole dietary system where people choose to eat certain kinds of foods together, and avoid certain combinations of food. With food combining, some people find that their digestion works a lot better.
How it works is they'll eat only certain foods together, like protein and vegetables, or carbohydrates, like whole grains and beans and vegetables, but they won't combine proteins and fats and starches all together. The reason that it works well for some people, they find that their digestion works better when they're not combining all of these things together, because the enzymes in the body are working on one specific kind of food at a time. That food gets digested at the right rate, and doesn't get inhibited by another food, which might go faster or might take longer.
If you have something like protein and fruit together, the fruit might digest really fast, whereas the meat that's fatty and heavy protein will take longer. Then you have two kinds of foods that are digesting at different rates. The fruit might be sitting on top of the meat, and it'll start to ferment in the stomach, and it will cause things like gas and bloating, stomach pain. Some people have constipation or diarrhea, so they find that when they do specific food combinations, their digestion works a lot better.