Short answers, pulled from the story.
AS-203 was launched on the 5th of July 1966 to study how liquid hydrogen fuel behaves in weightlessness inside the S-IVB rocket stage. Engineers needed to verify that anti-slosh measures were adequate and that engine temperatures could be maintained for a restart in orbit, a capability essential to sending Apollo crews to the Moon.
AS-203 deliberately flew without a command and service module so that residual propellants would remain in the tanks after the main burn, approximating the conditions the Saturn V would face in Earth parking orbit. An aerodynamic nose cone replaced the spacecraft payload.
After completing its primary objectives in the first two orbits, AS-203 underwent a closed fuel tank pressurization test on the fourth orbit. A pressure difference measured as high as 39.4 psi collapsed the common bulkhead separating the hydrogen and oxygen tanks, and NASA concluded that a spark or impact ignited the remaining propellants, destroying the stage. Because the primary objectives had already been met, NASA still classified the mission as a success.
The hydrogen behaved mostly as predicted during the first two orbits. The anti-slosh measures gave sufficient control over the fuel's location in the tank, and engine temperatures remained within the range needed for a restart in space.
In September 1966, Douglas Aircraft Company, which built the S-IVB stage, declared that the design was ready for use on the Saturn V to send men to the Moon. That declaration followed directly from the data AS-203 gathered during its first two orbits.
AS-203 launched from Pad 37B at Cape Kennedy, making it the first Saturn IB to lift off from that pad. The launch took place on the 5th of July 1966.