That is what you can get outside per plant in a meter sqaured place OR 500 grams in a meter sqaure closet no mater how many plants there are. In your meter sqaure closet you have another factor. You can expect .5 that's 1/2 gram per watt of light no mater how many plants you have. More than that s excellent.
I think they tell us this so we can look for high yeilds if we need them but some of the most sought after strains may be less producers.

Don't crowd your closet but get as many as you can in their and shoot for .5 grams per watt.