위험 = danger, hazard, risk. 위험성 = dangerous aspect or dangerousness, hazarddousness, riskiness. 성 is a suffix that indicates associated characteristic or property. By attaching it to a concept, you indicate an attribute or nature of the kind, rather than the concept itself. Adding the suffix doesn't change the overall meaning much, but adds finer detail and nuance. Examples. - 킥복싱은 부상 위험성이 큰 스포츠다 = Kickboxing is a sport with a high risk of injury. - 성실성은 중요한 덕목이다 = Sincere attitude is an important virtue.