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AS-202 S-IB/S-IVB Stage Separation.
The video is slowed down, in reality the time from the separation to the J-2 firing was under 4 seconds.
The S-IVB is the stage that put Apollo capsules on the lunar transfer orbit. Note that this is a Saturn IB upper stage, meaning this one never actually went to the moon, and only put the yet unmanned spacecraft on an suborbital trajectory.
The three small rockets are the ullage motors, small solid rockets used to create small acceleration, so the fuel settles own in the bottom of the tank and starts being fed into fuel pumps