Plate carriers are vests meant to hold hard plate body armor that would protect the vital areas on your torso. While some may have their own ballistic protection, most are solely for holding hard plates.
Materials inside a plate carrier can range vastly depending on the purpose of the plate. They can be very light, or very robust, and may or may not include additional features.
Although expected to be retired for a newer model in 2020, the US Armed Forces use the Interceptor Multi-Threat Body Armor System.