XR Stands for eXtreme Runtime. (may also be called extended runtime)
If it's stamped on the batteries: this is our claim that these batteries last longer than our non-XR versions. They are the higher amp hour batteries and have a larger fuel tank.
If it is stamped on the tool: this means that the tool is brushless. Brushless motors control energy transfer more efficiently allowing you to do more with the same amount of charge.