Imagine you have a gallon of water and put five tablespoons of salt in it, the resulting concentration would be five tablespoons per gallon (5 tbs/gal). Now imagine you have a two-gallons of water and add ten tablespoons of salt, the resulting concentrations would still be (5 tbs/gal). Although in the second case, the concentration of the pollutant is the same as it was in the first case, the total amount of the pollutant (load) is twice as high.
Consider now a stream in an area where flow is low during summer and high during winter. Let’s say winter flows are five times higher than summer flows and the nitrogen concentration is the same during the winter and summer. If you only considered the concentration you might conclude that there was no difference in pollutant levels through the year. However, there is five times more water in the stream during the winter, so there is five times more nitrogen being transported during the winter. The stream contributes five times the nitrogen load then it contributes in the summer, even though concentrations are the same.