"man" is your best friend.
#> man top
This line displays the time the system has been up, and the three load averages for the system.
The load averages are the average number of process ready to run during the last 1, 5 and 15 min*
utes. This line is just like the output of uptime(1). The uptime display may be toggled by the
interactive l command.
Also, should you ever need to or want to know the load average without running top, you can simply run the 'uptime' command.