I won’t be going into a whole lot of detail about sar as this has been documented elsewhere multiple times but basically, SAR stands for System Activity Report and as its name suggests, the sar command is used to collect,report & save CPU, Memory, I/O usage in Unix like operating systems. The SAR command produces reports on the fly and can also save the reports in the log files as well. The sar man page states:
The sar command writes to standard output the contents of selected cumulative activity counters in the operating system. The accounting system, based on the values in the count and interval parameters, writes information the specified number of times spaced at the specified intervals in seconds. If the interval parameter is set to zero, the sar command displays the average statistics for the time since the system was started. If the interval parameter is specified without the count parameter, then reports
are generated continuously.