This is a simple 4 cell AA battery holder. The 20CM (Approx) cable is terminated with a standard 5.5×2.1mm, center positive barrel jack connector.
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.
This pack also has a cover which you can lock using a screw (not included) and it has an On/Off switch which is super handy.
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 a lot of what board and processor you are using with the battery pack. Please consult your datasheet.