A question about a recipe: Retro Raspberry Lime Rickeys

I have a question about step 1 on the recipe "Retro Raspberry Lime Rickeys" from cheese1227. It says:

"Combine raspberries, sugar and zest in a saucepan and place over medium heat. Bring the mixture to a boil, stirring constantly until the berries dissolve and the sugar melts. Only let it boil for 2-3 minutes as you don't want jam. After that time, take the syrup off the heat and let cool slightly. Stain the syrup."