Why does the Strait of Hormuz matter?

After the US pulled out of the international nuclear agreement with Iran, Tehran said it will not negotiate another deal with Washington. There has been a deterioration in relations between the US and Iran since April, when the US ended sanctions exemptions, effectively banning all Iranian oil exports. Iran has threatened that if its oil exports are stopped, then no oil can pass the Strait of Hormuz. But, why is this strait so important? Parizad Nobakht from BBC World Service explains.