What does a soft box do?​

"Softboxes are also used for softening and increasing the size of a smaller light source, and diffuse light sources not unlike umbrellas—with a few notable differences. A softbox controls the shape and direction of light more than an umbrella and prevents more light-spill from occurring."

Explanation:

it helps reflect the light from the flash back onto the area that's being photographed. (there's other uses too but this is the only one i can remember)