Anything that's preventable can be avoided or stopped in its tracks. A preventable disease is one that you can easily be vaccinated against.
A lot of theft is preventable, if you remember to lock your car doors. Head injuries are mostly preventable too, if you wear a helmet when you ride your bike. Unless you're completely lost in your math class, bad grades on quizzes are preventable too; you just need to do your homework and study. When you prevent something, you keep it from happening. The Latin root is praevenire, "come before, anticipate, or hinder."