Most of the time, your Mac’s processor is using only a small percentage of its processing power. When you use apps that require intensive calculation, the processor uses more of its capacity.
To view processor activity over time, click CPU in the Activity Monitor window. The following percentages appear in the bottom of the Activity Monitor window:
System: The percentage of CPU capability being used by processes that belong to OS X.
User: The percentage of CPU capability that’s being used by apps you opened, or by the processes opened by those apps.
Idle: The percentage of CPU capability that’s not being used.
To view current processor activity, choose Window > CPU Usage.
To view recent processor activity, choose Window > CPU History.
To make more columns visible, choose View > Columns.