Body alignment is the optimal placement of the body parts so that the bones are efficiently used, so the muscles have to do less work for the same effect.
Pilates is a form of exercise that stresses proper body alignment. Many pains and problems in the body are associated with long-term mis-alignments. These can happen from bad habits or outside forces such as accidents or sports activities.
A good posture requires good body alignment.
References
Body alignment Wikipedia(Text) CC BY-SA