from top.c, line 39 (from busybox snapshot), there's a mention of /proc/loadavg
cp /proc/stat /proc/meminfo /proc/loadavg .
On my system right now, I see:
0.11 0.23 0.27 1/564 21830
RedHat Documentation explains it like this:
The first three columns measure CPU and IO utilization of the last one, five, and 10 minute periods. The fourth column shows the number of currently running processes and the total number of processes. The last column displays the last process ID used.