The photograph shows the SIVB stage of the Saturn V rocket following its extraction from the docked LM/CSM and the Earth horizon over the Pacific Ocean. A LM thruster quad can be seen in the foreground.

After docking with the LM-SIVB, the crew “performed for the first time the tricky maneuver of withdrawing the LM from the adapter on the Saturn booster’s third stage, where it had been sheltered during launch.” (Mason, pg. 152)

Following extraction, the Apollo 9 spacecraft was “to adjust its orbit 2,000 feet away from the S-IVB stage. The S-IVB engine was then to restart twice, placing the stage in an Earth escape trajectory and into solar orbit. This would simulate a translunar injection of the stage for Apollo 10 and subsequent lunar missions."

Read more: Apollo 9, NASA.

From the mission transcript following extraction of the SIVB stage:

004:10:43 McDivitt:

She’s right out there (the SIVB).

004:10:46 Scott:

She sure is.

004:10:49 McDivitt:

Heck, we’re going away from it, Dave. Good. [...]

004:11:45 Scott:

We’re in good shape on the controls; we’re in minimum impulse, and we’re stable. [...]

004:11:51 McDivitt:

Give me the Hasselblad, Rusty. [...]

004:11:54 Schweickart:

Okay, here you go. Should be all set up, Jim. [...]

004:13:34 Schweickart:

God, that’s beautiful! Could you see him real well, Jim? Because - He’s right out in front of me.