The connector mates with the barrel jack on the Arduino (among a number of other products) allowing you to easily make your project battery powered.

Note: the average voltage regulator has about 1V of dropout (but can vary greatly). This pack, with normal alkaline batteries, will output ~5.5V causing a normal 5V board to run at around 4 to 4.5V. This depends on a lot of what board and processor you are using with the battery pack. Please consult your datasheet.