Why do my braces need tightened?

During orthodontic treatment, the orthodontist stretches your jaw and moves your teeth so that everything fits.  If the orthodontist would try to move your teeth all at once, it would hurt too much and might damage the roots of your teeth.  The orthodontist moves your teeth slowly to avoid hurting you too badly.  Every time your braces are "tightened", your teeth move a little bit.  Then the orthodontist waits for your teeth, jaw & gums to shift, before moving your teeth again.