Risk is about future uncertainty. It is formally defined as the chance of something happening which will have an impact on objectives. It is measured in terms of the combination of the consequences of an event and their likelihood. This underlines the two dimensional nature of risk as shown in the graphic. There is another RiskBite with more about consequences
Likelihood is defined using either qualitative categories or quantitatively using probabilities. Probabilities are subjective numbers which represent our degree of belief rgarding which of the different consequences might materialise. The Risk Modelling RiskBite has more on this.