Rocoto is an Andean chili of the species Capsicum pubescens, often round and thick-fleshed with dark seeds. It is used fresh, in sauces and in stuffed preparations such as Peruvian rocoto relleno. Locoto is another regional name.
Despite a shape that can resemble a sweet pepper, rocoto can be very hot. It is not interchangeable with ají panca or bell pepper merely because the color matches. Handle it as a hot chili.
Reference: Further reading.