Rectifiers are devices that are able to convert AC(alternating current) input to DC (direct current) output. Rectifiers are useful when equipments need to run on direct current while we have to use wall outlet. For example, Laptop computers have internal circuit that can only run on direct current and we will need the rectifiers to convert AC to DC. That is why we have a thing called "adapter" for every laptop. In modern rectifier, special electronics flip the voltage on the current that is going up and down and make them only go up or only go down, then smooth out the whole current and make it the straight DC.