Vasoconstriction occurs when blood vessels become narrower because of the tightening of the muscles in their walls. Its opposite is vasodilation, which is when blood vessels widen. For example, when you are cold, blood vessels near the skin vasoconstrict so that you don't lose too much heat from your blood.
When blood vessels vasoconstrict, this increases blood pressure. Sometimes, blood pressure is too high and this can cause heart disease, so people have to take medicine to make the muscles in the blood vessels relax.