from man 3 getloadavg
The getloadavg() function returns the number of processes in the system
run queue averaged over various periods of time. Up to nelem samples are
retrieved and assigned to successive elements of loadavg. The system
imposes a maximum of 3 samples, representing averages over the last 1, 5,
and 15 minutes, respectively.
It's a way to determine how well the box keeps up
with its work, how many processes need to be waiting
until it's their turn... it's not exactly a measurement of CPU
usage, it also involves IO-waits ...