The brass grease cup is designed to replace the grease cap used on Freeman stern glands.
The grease cap unscrews from the base (which screws into the stern gland) so that it can be filled with grease then screwed back on top. Over time the cap should be tightened on to the base which releases the grease into the stern gland bearing.
The grease helps to lubricate the shaft in the stern gland bearing and is not designed to stop the water! If your gland is leaking it is likely the gland packing needs replacing.