The Prime and Fill tool is designed to fit snubly on the printhead of the 4500 and 45A cartridges from HP et. al.
Its purpose is to extract air and a little ink from the cartridge through the printhead thus to prime it. This system also helps rejuvinate the printhead.
The cartridge snaps into the tool and the syringe is inserted into the rubber. Wth the printhead facing up pull back on the syringe plunger to remove air and extract ink from the printhead.