Understanding system load

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http://unix.stackexchange.com – I was looking at the system load of my shared hosting account. $ uptime 09:14:37 up 21 days, 5:38, 4 users, load average: 3.85, 4.96, 5.21 I'm not worried by the first and second values, they represent peaks of work. But 5.21 for the last 15 minutes seems a bit high. Or maybe not? How do I know that? I'd like to read some answers to this last question. UPDATE: then, what is the unit of measurement of those numbers? (HowTos)