Atrun runs jobs queued by at(1).
Roots crontab(5) file /etc/crontab has to contain the line
*/5* * * * root /usr/libexec/atrun
so that atrun gets invoked every five minutes.
At every invocation, atrun will start all the jobs in the lowercase queues whose start time has elapsed. In addition, if the load average over the last minute was less than the specified limit then a maximum of one batch job (denoted by the uppercase queues) is started.