What does 'on thin ice' mean?

The term "on thin ice" means that someone is on the verge of getting in trouble, or something bad happening. Thin ice is dangerous to stand on, and if you walk or skate on it, you will fall through the ice into the freezing water below.

For Example: She was on thin ice when her mother saw her report card.

You are taking a chance and doing something that could get you in trouble.
its used to describe a person when making impulsive decisions which would have an adverse impact on him.

This means that you are walking on unsafe ground and your about to get in trouble.

It means that you should be careful with what you do.

You are taking a chance and doing something that could get you in trouble.

The image is of a person standing, walking, skating, whatever -- and they come to a patch of thin ice that could crack and let them fall into the water.
Walking on thin ice means that you are taking a risky chance on something, and before you do it, you are already aware of the risks involved.Usually involving relations with another person, not taking risks such as jumping out of a airplane.