The first set of values come from PHP function posix_times(), so you can read their description from http://www.php.net/manual/en/function.posix-times.php, for example. Just a minor note: instead of absolute values, Moodle prints the difference between the startup values (i.e., the values present at the beginning of the page execution) and the final values.
The load average is a well-know Unix concept (and is in fact only shown when it runs on a Unix-like OS). Moodle just shows the last minute load average.
Saludos. Iñaki.