Overview
In UNIX computing, the system load is a measure of the amount of computational work that a computer system performs. The load average represents the average system load over a period of time. It conventionally appears in the form of three numbers which represent the system load during the last one-, five-, and fifteen-minute periods.
Unix-style load calculation
All Unix and Unix-like systems generate a dimensionless metric of three "load average" numbers in the kernel. Users can easily query the current result from a Unix shell by running the uptime command:
$ uptime
14:34:03 up 10:43, 4 users, load average: 0.06, 0.11, 0.09
The w and top commands show the same three load average numbers, as do a range of graphical user interface utilities.
In operating systems based on the Linux kernel, this information can be easily accessed by reading the /proc/loadavg file.
To explore this kind of information in dept, according to the Linux's Filesystem Hierarchy Standard, architecture-dependent information are exposed on the file /proc/stat.
From Wikipedia (CC BY-SA 4.0).