Hi mate, the only silly question is the one gone unasked

PPM is a measure of concentration and means Parts per Million. so that one kilogram of salt dissolved in 1 million liters of water is 1 ppm. 1 liter of water weighs 1 kilogram so the weights are apples to apples.

A million liters is a lot so lets divide everything by 1 million:

1000grams/1,000,000= 1miligram salt

1,000,000 liters/1,000,000= 1 liter water = 1 kilogram of water.

So , 1 mg of salt dissolved in 1 liter (1 kg) of water is also 1 ppm.

Hope this makes sense.

Cheers,