A wort chiller is equipment used to cool freshly-boiled wort down to yeast-pitching temperature quickly — commonly an immersion coil submerged directly in the kettle, a counterflow chiller running hot wort and cold water through concentric tubes, or a plate chiller using stacked metal plates for a compact, high-surface-area heat exchange.
Speed here isn't just about convenience
Rapid chilling matters for more than saving time waiting around after the boil: cooling quickly produces a stronger cold break (proteins and polyphenols precipitating out as the wort drops through a specific temperature range), and it minimizes the window during which unprotected hot wort is exposed to airborne contamination before yeast is pitched and active fermentation can start defending the batch itself.
Different chiller types trade speed, cost, and cleaning effort against each other
An immersion chiller is the simplest and cheapest to build or buy, but generally the slowest of the three common types and still needs a full kettle's volume to cool. A counterflow chiller cools faster and works well with continuous-flow setups but is harder to clean thoroughly since its internal passages aren't directly visible. A plate chiller cools fastest of all in a compact footprint but carries the highest up-front cost and, like a counterflow chiller, needs careful cleaning of internal channels after every use.