A "spreader" is a device that keeps the legs of a tripod from falling out flat like a duck on ice.

They are more commonly used with higher-end tripods. That's because cheaper tripods have a built-in spreader that limits how low you can position the tripod.

The spreader at the right--like most professional spreaders--is made of a thick flexible rubber. And it can be extended depending on how low you want to position the tripod head.