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.
For potatoes, rub cooked potatoes against a colander.