thats for salt water setups..
you dont NEED to add salt to freswater... thats why its fresh water... however it does help in healing (wounds/disease) and such... it can be hell on plants and some fish dont do well with it...
aquarium salt, kosher salt, the blue bag of mortons water softner salt... prettymuch all the same stuff..
i only use salt in my feeder breeding tank and arowana/datnoid tanks (for some reason the aros and dats appear to enjoy having a bit of salt in the water).. from time to time i will add a bit to other tanks if a fish has managed to beat itself up..