The Rocket Garden is where Starship prototypes are stored during times when space at the Construction site is limited, or a prototype is due for transport to other locations at Starbase, or the prototype is undergoing final preps for a flight test.
Currently the Rocket Garden is occupied by Ship 20 (S20), Booster 12 (B12) and Booster 17 (B17), but was occupied formerly by many other notable vehicles such as SN15, SN16, S22, S24, Ship 26 (S26) and Booster 8 (B8) which were all subsequently retired and scrapped (or blown up in flight in case of S24). Booster 4 (B4) was stored at the rocket garden for an impressive 630 days before being moved into Mega Bay 1 for scrapping, however, S20 (the ship that did a full stack with B4, is still there.