Risk is the possibility of something bad happening. People use the word risk when they are actually talking about probability; it means, they want to say how likely something is to happen.
People often take risks. This means that they do something that is risky, i.e. that might be dangerous. There is a possibility that something bad might happen.
In epidemiology, risk is defined as the number of events divided by the number of people prone to that event. For example, if the risk of neonatal death in a population is reported as 1/1000, it means that from each 1000 babies born, 1 is likely to die.
Risk is different from prevalence. Prevalence refers to the number of people with a disease in a population, while risk means the number of new cases, divided by the number of people who could possibly become a case (for example, those who had a chance to be exposed to the disease).
A risk assessment is a list of things that could go wrong. In many Western countries, when teachers take children on an outing they may have to fill in a risk assessment form which lists all the possible dangers and says what they should do to stop bad things from happening (e.g. accidents or children getting lost).