Silverstang
It is called the evaporator/or condenser. Is for your A/C.

gdwrnch40
Condenser

Mike M
Like Silversta said it's the condenser for your air conditioning system and no, the plastic fill bottle won't make it anymore difficult to change the radiator.Just be sure that you don't damage the AC condenser while taking out or putting in the radiator.

mister ss
the radiator in front is for the air conditioning and no , having to fill your radiator from the overflow won't make the job harder, the only thing that might be different is you have to bleed the air out of the system before the engine will take the anti-freeze from the radiator. there should be a bleed valve someplace on your engine for this.