Add water and brown sugar to a medium saucepan and bring to a boil over medium-high heat.
Once boiling, lower heat and let mixture simmer for about 10 minutes, until sugar is completely dissolved and syrup has thickened a bit (it will continue to thicken as it cools).
Remove saucepan from heat and pour syrup into a heat-proof bowl or mason jar. Place bowl or jar in fridge to cool completely before using.