These are practice contractions that usually start around the 20th week of pregnancy. Your uterus is flexing its muscles, warming up in preparation for the birth.
These contractions are usually painless but they can be uncomfortable. The uterus tightens at the top and gradually spreads downward before relaxing. They usually last between 15-30 seconds but for some as longs as 2+ minutes.
As your pregnancy draws towards the end these contractions may become more frequent, intense and perhaps more painful. These early contractions are not efficient enough to deliver your baby but they certainly may be a sign that early dilation of the cervix has started. Use this time to practice the labour techniques you have learnt.
Do however keep your midwife or doctor informed and report these immediately if they are frequent (more than four per hour) and/or are accompanied by pain (back, abdominal, or pelvic) or any kind of unusual vaginal discharge, or a history or a warning regarding premature labour.