On display at the San Diego Air and Space Museum, it’s the Apollo 9 Command Module:
Launched on March 3, 1969, Apollo 9 was the third manned Apollo flight, the second manned launch of a Saturn V rocket, and the first flight of the Command and Service Modules (CSM) with the Lunar Module (LM) — so, the first flight of a complete Apollo spacecraft stack
During 10 days in low Earth Orbit, the Apollo 9 crew would perform the first manned flight of a LM, multiple docking and undocking exercises with the CSM and LM, navigation and backpack life support systems, and a variety of other exercises designed to prove the LM worthy of manned spaceflight.
Want more background? Here are some great links: