To pinpoint something is to nail down its exact location.
The point of a pin is very small and fine. Similarly, when you pinpoint something, you have narrowed down its location to the smallest possibility: you've found out exactly where it is. The government is constantly trying to pinpoint the location of terrorists, just as police try to pinpoint the location of criminals. Dogs can pinpoint many things through smell. It's often hard to pinpoint which car is producing an annoying car alarm. When you pinpoint something, you've found it.