Braxton Hicks contractions (name courtesy of the English doctor who first described them in 1872) are relatively painless, irregular, and infrequent contractions that start around week six and lasting until the weeks before delivery. Though they start in the first months, you probably won't feel them until around week 28 (if at all). These contractions are your body's way of practicing and preparing for labor.
The tightness you feel in your uterus during a Braxton Hicks contraction comes from the tensing of the muscles in your uterus. Although they cause more annoyance than pain, drinking water or taking a walk or a warm bath should relieve any discomfort. Mild Braxton Hicks contractions are very common and nothing to worry about, but call your doctor if you are not yet full term and feel four or more contractions in an hour — this might be a sign of preterm labor.