An OTDR, or Optical Time Domain Reflectometer, is a device used to test and analyze fiber optic cables. It sends a pulse of light into the fiber and measures the light that is reflected back. This helps identify faults, breaks, or bends in the cable, as well as determine the overall quality of the fiber. OTDRs are essential tools for telecommunications and networking professionals, as they provide detailed information about the length and condition of fiber optic links. By using an OTDR, technicians can quickly locate issues and ensure optimal performance of fiber optic networks.