Standing post (站桩 zhàn zhuāng) is a meditation and posture correction exercise. As it is an externally static posture, it is a useful way of beginning to learn how to quieten your mind and develop an awareness of your body.
The main requirement for a correct posture in taijiquan is that the dantian (丹田 dān tián), roughly the centre of what is called the core in English, should feel warm and full. The dantian is located a few inches below the navel and inside the body. In order to allow this area to fill up with energy all the muscles driving the joints in the body, especially the hips, should relax and loosen and the mind should remain calm. Only the minimum muscle activation required to remain standing should be present. Doing this requires the development of strength in the thighs, in particular three of the four quadricep muscles (the other one crosses the hip joints and is therefore also a hip flexor) must become strong enough to support your body weight. This allows the top half of the body to relax and sink down. For more information, see Daniel Poon’s page on the body requirements of taijiquan.