A salad spinner is a simple device: a kind of mesh basket which is spun by turning a handle on the lid. As the basket turns very fast, any items in it are forced agaisnt the sides by centrifugal force, and the water (or other liquid) they contain is spun off and collects in the bottom of the container.
Of course, it's designed for drying salads, but it works well for other things, like cooked spinach for example.
If you intend to buy:
Choose a model that is robustly made, especially the handle, which has to withstand a lot of force.