"Risk of mortality" means how likely it is that someone might die. It's like playing a game where you have to guess if someone is going to win or lose. If the person is very healthy and doesn't have any problems, they have a low risk of mortality, or a low chance of dying. But if the person is sick or has a lot of problems, they have a high risk of mortality, or a high chance of dying.
Doctors use something called a "mortality rate" to figure out how risky it is for someone to die. The mortality rate is the number of people who die out of a group of people who have the same problem. For example, if 10 out of 100 people with a certain sickness die, then the mortality rate is 10%. So if you have that same sickness, your risk of mortality or your chance of dying is 10%.
It's important to know about risk of mortality because doctors need to figure out how to help people who might be at a high risk. They might give them medicine or do tests to try to make them feel better and to make sure they don't die. And you might want to know about risk of mortality too, so you can be healthy and take care of yourself, so you can live a long and happy life.