A small snack shortly before you exercise may actually be a good thing, or a high energy drink. A more substantial meal about two hours before exercising is a good idea too; in fact if you do hard exercise - like running, for examples - on an empty stomach you may well get sick and dizzy due to low blood sugar.

However, eating a full meal less than two hours before isn't a good idea, and if it's really close - say, you have dinner and then get straight up and go for a run - you will probably feel very uncomfortable and may even get nauseous. One reason you shouldn't eat too much before exercise is that you are still digesting, and the energy you will get from the food isn't available yet (that's why people often get sleepy after a meal.) So you won't perform at your best.

You can find sensible food and exercise advice here.