It's a nice 5-AA battery holder, with 5" leads and a 2.1mm DC barrel plug.
What's to say? It's a holder for five AA batteries. Oh, and it has 5" wire leads. Did we mention it's black? And that it has a 2.1mm barrel plug (tip positive). You might find that important...
These battery packs are common for powering Arduino boards and other development boards because they usually provide upwards of 7.5V (with alkaline batteries installed) which is perfect for the onboard 5V regulators.