OK, I have made a lot of research to answer your question. The (average) resource load factor is not a standard project management term, and it was mentioned (in passing) in only couple of books. The average resource load factor can be calculated the following way:
ARLF = ARR / ARA
Where ARLF stands for Average Resource Load Factor, ARR stands for Average Resource Requirement, and ARA stands for Average Resource Availability.
For example, if during a six months period, a resource is allocated 80 hours/month on average, and is available for 200 hours/month on average, than the ARLF = 80/200 = 0.4, which means that the average load factor of the resource is 0.4 (in other words, the resource is allocated 40% of the time)