A ricer is a device with small holes in it through which you press soft foods. The food extrudes out through the bottom of the holes in strands about as thick as a grain of rice, thus the name. The process leaves skin and seeds behind.
Some are completely manual, like a sieve in an inverted cone shape, and you work a pestle (usually made from wood) round and round in them to force the food item through. These are called cone, conical or rotary ricers.
Some are somewhat mechanized, with a handle that you turn or squeeze. Ones with handles that you squeeze together are specifically known as potato ricers.
Most ricers are made of stainless-steel, though some are made out of aluminum. The stainless-steel ones are more likely to be dishwasher-safe, but check before you buy.
You can purée apples through one, or use it instead of a potato masher to make mashed potato. It doesn’t cause the starch in foods like potato to become glutinous. You can also use it to press spinach dry.
A ricer is similar in appearance to a chinois strainer.
For potatoes, rub cooked potatoes against a colander.