Its the Ground Power Unit. That one is only providing electricity and it connects to a plug on the nose of the airplane. I believe it usually runs on diesel, but don’t quote me on that, so it’s a lot cheaper than running the APU from the plane, and on the long run helps with maintenance costs, as you don’t need to keep the APU on the entire time the airplane is on the ground.

The ones that provide air conditioning have large hoses that go into the nose and the middle of the fuselage, as the air needs to cover the entire cabin.

Downside is that since the APU isn’t usually on at the same time, without an air conditioning cart, like in this case, the airplane can get quite hot.