You have found the recipe for double strength simple syrup. This is a simple syrup made from 1 part water and 2 parts sugar by volume. 2:1 syrup is the one I make and keep on hand in my bar. It takes up less space than the single strength variety and you can just reduce the amount you use if a recipe calls for 1:1 simple syrup.
If you are wondering what strength the simple syrup is that you just bought in the store then look at the calories. 2:1 syrup is about 90 calories per fluid ounce.
Heat sugar and water to boiling. Boil for about 1 minute while stirring. Turn off the heat and let cool to near room temperature. Stir in the rum and bottle.
The rum is here to help preserve the syrup. I store mine in the refrigerator and it keeps for months. This simple syrup recipe makes about 16 oz of syrup.
There is some raw sugar in the recipe to give the simple syrup a little character. Just a little and not too much because this syrup is meant mostly to sweeten drinks and let the other ingredients provide the flavors.