When you eat, so does the bacteria inside your mouth. In particular, it thrives on sugars and starches: cereal, bread, crackers, candy, and cookies. The bacteria then forms an acidic film over your teeth called plaque.
When you brush your teeth regularly and frequently with a soft brush, your goal is getting rid of the plaque and any food particles that may have gotten stuck between your teeth. If you don’t brush, though, the plaque stays on your teeth for an indefinite amount of time. And because it is acidic, it eats away at your tooth enamel a little at a time. It also irritates your gums. You can predict the result: cavities. Plaque is especially adept at eating away where you are already vulnerable; around fillings and amalgams, where you are most likely to have dental weaknesses already.
If you had a slice of lemon against your gums, you wouldn’t be surprised if the acid in the lemon made your gums sore and irritated after a while. It’s no different with plaque. Leave the plaque there long enough and you are also more likely to bleed. The gums try to pull away from the plaque because it is so irritating, but that means pulling away from your teeth as well, since that is where the plaque is. Dentists call this problem receding gums. The area around the gums can become infected, so that you have little pockets of pus between your gums and your teeth. This serious problem results in bone loss and, eventually, tooth loss.