Image via Wikipedia vmstat provides a summary of various functions within the system, including system wide free memory, paging counters, summarized disk activity, system calls and cpu utilization.
The output of vmstat and description of what each field means:
The first line of output from vmstat shows a summary since boot,
followed by the output over the last 3 seconds for each additional line.
The vmstat command reports the amount of swap space that is free (not reserved or allocated).