Tar is the yucky, sticky, brownish-black goo that's left behind when you smoke cigarettes or tobacco. Think of it like the leftover gooey stuff in the bottom of the sticky jar of honey.
When you light up a cigarette or tobacco, it produces a smoke that contains lots of different chemicals. Some of these chemicals are called "tar" and they stick to your lungs and throat.
Tar is bad for your body because it can cause damage to your lungs and can make it hard to breathe. That's why it's important not to smoke and to try to avoid being around other people who smoke too!