In NYC, there is something called "Alternate Side Of The Street Parking." This means that it is illegal (a parking ticket offense and possible towing) to park on one side of the street on Mondays, Wednesdays and Fridays, and on the other side Tuesdays and Thursdays. The ostensible reason for this is so that the street cleaners can clean each side of the street every day; the real reason is to generate parking ticket/towing revenues for the NYC government.
Sid's job, which George did incompetently while Sid was away, was to move the cars for their owners from one side of the street to the other every day, so the owners could avoid being ticketed and/or towed. Hence the title, "The Alternate Side."
Although I no longer live in New York, I remember how my mother used to curse having to "move the car" every night. It is one of the most loathed aspects of living in NY, and certainly one of the most frustrating laws.
This was actually a very good question, since it points out one of the unique NYC idioms used in SEINFELD that viewers in other parts of the country would have no way of knowing. It really shows how much a "New York" show SEINFELD is (was), and how unusual it is that it became such a universally appealing show.