A contraction is when your womb tightens and then relaxes. If you put your hand on your abdomen you can feel it getting harder and then as your muscles relax and the pain fades, it would get softer.
Contractions are the real sign that you're going into labour. You might have felt Contractions during pregnancy, especially as your due date grew closer. These are called Braxton Hicks Contractions.
When you start having regular, painful, strong Contractions that last for more than 30 seconds and come about every five minutes, labour may have started. You should call your midwife or the hospital.