Barrier Islands
Barrier islands form when ocean waves/currents repeatedly deposit large amounts of sediment parallel to shoreline. The process occurs over the course of thousands of years, and results in long, narrow islands that shield the mainland from waves.
Barrier islands are only found where there is abundant sediment supply, and a shallow coastal slope e.g. close to deltas, and not along active subduction zones. This is why you're more likely to find barrier islands in the US along the East coast or near the Gulf of Mexico.