Before understanding what is risk based testing you should know what a risk is. A risk is something that has not happened yet and it may never happen in future as well, however, it is the potential problem.
Risk Based Testing
Risk-based testing uses risk to prioritize the appropriate test cases during the project. It starts early in the project cycle. The risks to system quality are identified and that knowledge is used in the test planning, preparation, and execution.
Risk-based testing is testing efforts in which we try to reduce the level of product risk when the system is deployed.
Risk-based testing starts early in the project, identifying risks to system quality and using that knowledge of risk to guide testing planning, specification, preparation, and execution.
Risk-based testing involves both mitigations – testing to provide opportunities to reduce the likelihood of defects, especially high-impact defects – and contingency – testing to identify workarounds to make the defects that do get past us less painful.
Risk-based testing can also involve using risk analysis to identify proactive opportunities to remove or prevent defects through non-testing activities and to help us select which test activities to perform.
How to perform risk-based testing?
Make a list of risks base on the Risk Priority.
Perform testing that explores each risk.
As risks evaporate and new ones emerge, adjust your test effort to stay focused on the current crop.