What is a speedring used for?

Softboxes are comprised of multiple metal rods, an external fabric (usually with silver material inside), and one or two diffusers that sit inside the softbox to soften the light.

But how do you actually put all that together? Another item called a speedring is required. 

The rods of the softbox are fed into the external fabric to form the ribs, and the ends inserted into a Speedring to create tension in the frame like a tent. Then the opening of the speedring is attached to your preferred light.

Profoto makes speedrings to adapt the RFI range of softboxes to a number of major flash head brands.