The load average given by Unixen expresses, basically, a percentage of how much CPU time is being utilized by running processes. There are many processes (such as init) which rarely ever need CPU time. When all processes are in this wait state it is quite possible to have a load average of 0.00.

The ideal situation is 1.00, though. This means that every CPU cycle is used, but no processes are queued, waiting for time when they would normally be receiving it. You might get this when running SETI@Home on an otherwise-idle box, because SETI@Home will use additional CPU cycles only until the load is 1.00.

When the load exceeds 1.00, you have all CPU cycles in use, and processes queued waiting for time, rather than getting it right when they need it. This is when CPU lag occurs. A load of 2.00 means there are just as many processes are waiting for CPU time as there are processes receiving the time they need. 3.00 means there are twice as many waiting as there are running. 4.00 means three times as many, and so on.