One way of characterising the energy output of a power plant over a period of time (typically a year) is to state how many hours the plant would have taken to produce this amount of energy had it been operating at full capacity (maximum power output) for that time. This is the number of full load hours.
Like capacity factor therefore, full load hours is another way of expressing the relationship between capacity, energy and time for a power plant. The relationship between the two is that full load hours = capacity factor x full period (hours).
For example, consider a 100MW power plant that generates 438,000MWh over the course of a year (8760 hours).
Capacity Factor = Energy Generated / (Capacity x 8760) = 50%
Energy Generated (MWh) = Power (MW) x Time (h).
So if power output was constant, at the maximum rate (capacity) of 100MW and energy generated is 438,000MWh, that can be rearranged to give time = 4380 hours.
4380 hours = 50% x 8760 hours (half a year).
In other words, a power plant that operated at a capacity factor of 50% over the course of a year could have produced that same amount of energy had it operated at full capacity for half of the year (4380 full load hours).