it is used for making the tank Brackish. it increases the salinity. It goes from fresh, mild brackish, mid brackish, high end brackish and then Marine.
so unless you are keeping brackish species it will be damaging to your fish as some fish can tolerate brackish water (some cichlids can even live in marine conditions for extended periods) fish such as pleco's do not tolerate salt well.
If you are planning on using it to medicate your tank thats what aquarium salt is for. it will not alter the salinty of your tank.