Combine brown sugar, raisins, vinegar, ginger,capers, red pepper flakes, and black pepperin a medium skillet. Cook over medium heat,stirring often, until liquid is reduced by half,about 5 minutes.
Add rhubarb to skillet and stir to coat.Cook, swirling pan occasionally, untilrhubarb is tender and liquid is syrupy,about 15 minutes.
Serve compote warmor at room temperature.
DO AHEAD: Compote can be made 5 daysahead.
Let cool; cover and chill. Reheatslightly, if desired, before serving.