Apollo 5 was the first unmanned flight of the Apollo Lunar Module, which would later carry astronauts to the lunar surface. It lifted off on January 22, 1968 with a Saturn IB rocket.
The Apollo 5 mission tested the Lunar Module in a space environment, in particular its descent and ascent engine systems, and its ability to separate the ascent and descent stages. The descent engine would become the first throttleable rocket engine fired in space.
The mission also performed a "fire in the hole" test (as depicted in the mission's insignia) in which the ascent stage engine would be fired while still attached to the descent stage. This was intended to simulate a landing abort during descent to the lunar surface.