A softbox fits around the head of a flash to provide a larger and more diffuse rectangular light source. It’s very popular amongst professional photographers for product shots, where it produces even lighting and nice reflections off glossy surfaces, and for portrait photographers who want to achieve a softer, more flattering effect.
Softboxes fold flat for easy storage and transport. The inner surfaces are highlight reflective and direct the light efficiently towards the diffuser panel on the front. They’re standard kit for professional flash systems.