Under CPU, you can get a clear and summarised version of the information and level of processes taking place on your CPU. It is best to have lower User and System usage in percentage form, and higher Idle usage to ensure your fans and Mac is running smoothly. If your User and System percentage is very high, open Activity Monitor on your Mac and Quit any process that seems to be using far too much CPU power.
Uptime shows how long your Mac has been running since it was booted last. Lastly, the graph shows an interval of the User and System stats of your CPU. Red = User info and Blue = System info. You can adjust the interval speed of the graph from the Preferences menu of the app under General (open the Preferences using the gear icon on the top right corner).