The Post Glacial Rebound (PGR) is rebound or bounce back of the land due to continetal ice and glacial melt that occured thousands of years ago. The PGR is calculated using the ICE-5G model run over thousands of years, resulting in a linear trend that is used for the PGR.
This correction has already been applied to the TELLUS products. If you would prefer to apply your own PGR or study just the PGR it is provided. For more information please visit http://grace.jpl.nasa.gov/data/get-data/gia-trends/