A speed ring is a solid metal ring-shaped disk which is used to attach a soft box to a studio (strobe) or continuous (hotlight) light source. There are 4 to 8 holes drilled into the circumference of the ring which receive the metal rods of the soft box and are held in place by tension. Shapes can vary depending on the manufacturer.
Typically the opening in the center ranges from 4 to 7 inches, allowing the light source to enter the softbox or other modifier. Common ring types are Bowens, Profoto and Alien Bees.
References
Speed ring Wikipedia(Text) CC BY-SA