>>> uptime
09:20:04 up 13:22, 5 users, load average: 0.11, 0.31, 0.42
Load average is those three numbers. It gives an idea about how busy the system is.
It shows load for last 1, 5 and 15 mins, A common misconception is that it just shows how business of the CPU. It also considers other IO operations.
Read the post below that explains when and how it is the way it is.