On the one hand, this isn't news: the report of the World Commission on Dams, published in 2000, notes on page 75 that "the gross emissions from reservoirs may account for between 1% and 28% of the global warming potential of GHG emissions." On the other hand, this new study finds that emission of gases may be 25% higher than previously noted.
What goes on is that reservoirs, usually made by damming up a river, end up submerging a great deal of plant life. In China, the Three Gorges Dam, completed in 2012, flooded 244 square miles of land, including 100,000 acres of productive farmland which used to produce 10% of China's annual grain. As I understand it, submerged plants basically rot, consumed by bacteria and turned into methane, which adds to the burden of greenhouse gases emitted every year.
A few caveats here: first, it's not all about dams. Other types of reservoir can do the same damage. Second, obviously dams do a lot of good for the climate as well: that same Three Gorges Dam produces nearly 100 TW per year, Producing that by coal plants would release at least billions of tons of CO2 into the atmosphere, based on numbers listed here. Averting that is definitely a good thing, though it's not free!