When making a custard (which serves as the base for this ice cream recipe), it is necessary to scald the milk. This accomplishes two things: it dissolves the sugar, and when whisked into eggs it increases their temperature slowly and helps prevent curdling.
To scald milk, simply place a medium saucepan over medium heat. Add milk, stir occasionally, and watch for steam and little bubbles to appear around the edges of the pan. Do not let the milk boil. Remove the pan from the heat and proceed with your recipe.