help time in bash gives:
time: time [-p] pipeline
Report time consumed by pipeline's execution.
Execute PIPELINE and print a summary of the real time, user CPU time,
and system CPU time spent executing PIPELINE when it terminates.
man 7 time give a description of
user CPU and
system CPU time:
Real time is defined as time measured from some fixed point (e.g., the start) in the life of a process (elapsed time).
User CPU time is the time spent executing code in user mode. System CPU time is the time spent by the kernel executing in system mode on behalf of the process (e.g., executing system calls).