A potato ricer is a kitchen tool that forces cooked potatoes through a bunch of small holes.  It works similarly to a garlic press, just on a larger scale.

Why would you use a potato ricer?

Pressing chunks of boiled potato through a ricer breaks them down into small clumps that can be easily mashed.  Because the potatoes haven’t been beaten with a mixer, they are lighter and fluffier (as long as you don’t over-mash them after ricing).  Try using a ricer with heirloom potatoes (particularly German Butterball potatoes or Yellow Finn potatoes) to really take your mashers to the next level.

Many chefs prefer to use a food mill for the same task, as it also has a variety of other uses in the kitchen.