Parts per million (ppm), means how many liters of substance do you have per million liters of water. Or, how many mL of substance per million mL, which is the same as per 1000 Liters. Water weights one gram per mL, so you can also call that how many grams of substance per 1000 liters of water. Again, you can divide both of those by a thousand, and you get how many milligrams of substance per liter of water.
If you have a 100 liter tank and want 10 ppm of table salt in it, you convert 100 liters to a mass measurement (in your head), which is 100,000 grams. Ten ppm would require .1 grams of table salt, which is 100 milligrams. So, measure out 100 milligrams of salt and dump it into the 100 liters of water and you have 10 ppm of salt in the water.
Take two aspirin for the headache I caused....sorry about that.