Ew, cavities! I worked hard for my first one. Forgot to floss for a few months (I did brush), ate a TON of sugar and didn't see the dentist for 1 year.
What did I learn? That toothbrushes don't clean between teeth (which is why we need to use floss), so hungry bacteria already hanging out in my mouth started feasting. One consequence of bacteria feasting is acid. This acid slowly dissolves the tooth. If caught early enough, it only dissolves the outer part of the tooth, called the enamel. If you don't see the dentist for years, though, the acid might get all the way to the inside of the tooth, the dentin -- which means Big Trouble: fillings, toothaches, crowns or even root canals! Unless you really want that gold tooth.
So what they say is true: floss every day, don't eat candy instead of meals, and see the dentist every 6 months -- Or you will have a reason to be afraid!