Setting postures refers to the practice of correcting the moves of the Tai Chi form by extending the concept of Settling, using guidance from your instructor.
Your instructor is a valuable resource who can save you weeks and months of time by guiding you in the setting process. He will do this by making minute adjustments to your form as you stand in a posture and you must rigorously attend to those adjustments until they become natural to your posture for that move. Basically any kind of adjustment that your instructor does will contribute to the setting process.
Also see Settling.