The Sanskrit word tada means mountain. This posture is also known by the name samasthiti-asana. Sama means unmoved, equilibrium, and sthiti means standing upright or firmly, abiding, remaining, thus samasthiti means standing firmly without moving.

1 Stand with both feet touching from the heel to the big toe, keeping the back straight and the arms pressed slightly against the sides with palms facing inward.
 
2 Slightly tighten or flex the muscles in the knees, thighs, stomach and buttocks maintaining a firm posture. Balance you weight evenly on both feet.
 
3 Inhale through the nostrils and lift the buttocks off the legs arching the back and thrusting the abdomen forward and tilt the head as far back as possible.

Many common ailments and discomforts can be traced to poor posture. If the spine is not properly aligned or if there is tightness or stiffness in the back, the result is often an imbalance in the body. When this imbalance becomes chronic many kinds of disorders arise in the organs, glands and nervous system.
Performing the tada-asana allows one to observe one’s posture closely and clearly recognize those problems which get masked or ignored by day-to-day activities. As the posture is held and the breath, mind and body is quieted various effects will surface to indicate difficulties with the spine. Favoring one foot over the other, shifting back and forth, drooped shoulders, tightness in the neck and upper or lower back, and various other physiological disturbances may appear indicating the need for further yoga practice.

The proper execution and continual practice of the tada-asana along with other postures helps to re-train the body to stand correctly and reverse the negative effects of poor posture.

When the tad-asana is performed properly and the mind is focused and free of distraction, the body is experienced as being rooted firmly to the earth and as steady and motionless as a mountain. Source