It's Poole's birthday; his family recorded a greeting for him on time delay, since he's too far from earth for instant communication.
Hal says there's a problem (you don't want a problem when you're in deep space. Problems are bad.)
There's something wrong with the AE-35 unit.
What is the AE-35 unit, you ask?
Arthur C. Clarke will tell you.
The AE-35 unit is the doohickey that controls the gizmo that keeps their communications doodad focused on earth. If the doohickey goes on the fritz, they won't be able to get messages from earth, which would be really unfortunate.
Also unfortunate—the AE-35 unit is outside the ship, so somebody has to go outside to fix it.
The astronauts have some back and forth with earth, and there is additional technobabble.
But basically, the message is, Poole is going outside the ship to fix the AE-35 unit.