Danger is a situation in which there is a high risk of something very bad happening. A situation like that is dangerous.
There are many situations which can be described as dangerous. It is dangerous to cross the road without looking carefully because you might get hit by a car. Some animals are dangerous because they might attack humans.
Anything that involves a risk of injury or to health can be described as dangerous. Smoking is dangerous to one's health.
Someone who is "on the danger list" is so ill that they might soon die.
Another word for danger is peril.