Please read the following „Basic Concepts“ section in order to get a clear overview of the basic premises required for the proper operation of PRTG Traffic Grapher.
Basic Concepts: Understanding Views, Sensors, Graphs, Charts, Tables, and Tags
Starting PRTG for the first time
PRTG Traffic Grapher is an easy to use Windows software for monitoring and classifying bandwidth usage. It provides system administrators with live readings and long-term usage trends for their network devices. The most common usage is bandwidth usage monitoring, but you can also monitor many other aspects of your network like memory and CPU utilizations.