This entails “dumping” your reservoir and re-filling it with fresh water and nutrients. The reason for this is that as the plants feed, the nutrient solution will fall out of balance. Also, bacteria grows at a geometric rate. If you change your solution every week you will decrease the possibility of bacteria becoming a problem.
While it is possible to go longer between changes if you are using reverse osmosis water instead of tap water, you still have the bacteria issue to contend with, so unless you are using something to inhibit the bacterial growth, you should still change your reservoir weekly.