Place 2 cups chopped rhubarb, 2 cups water, and 1 cup granulated sugar in a large, heavy bottomed sauce pan. Place over medium heat and simmer on medium-low for 15-20 minutes, stirring occasionally, until the rhubarb releases its juices and has become very soft.
Remove from heat and set aside to cool for 10-15 minutes. Pour the the syrup through a fine mesh strainer into a bowl, using the back of a wooden spoon to press it down to remove as much of the juice as possible.
Notes
Dissolve the sugar fully. For the smoothest syrup, make sure to fully boil and dissolve the sugar so it's not grainy.
Make sure to strain out the rhubarb pulp. This step must NOT be skipped; otherwise, you'll have more of a rhubarb compote situation, which, while lovely, is not simple syrup.
Double it. Love this syrup? Easily double or even triple this recipe.
Freeze it. You can freeze the syrup too - see how to do this lower down.
Can it. Follow the preserving guidelines from the National Center for Home Food Preservation at the University of Georgia to preserve the syrup for a longer time.