An immersion chiller is a coiled metal tube (usually copper or stainless steel) submerged directly into the hot wort at the end of the boil, with cold water run continuously through the coil's interior to draw heat out of the surrounding wort — simple, relatively cheap, and the most common first chiller a homebrewer buys or builds.
Why it's the easiest chiller to sanitize confidently
Because the coil itself is fully visible and physically accessible, an immersion chiller is straightforward to inspect and sanitize between uses — a real advantage over a counterflow or plate chiller's enclosed internal passages, which can't be visually checked for residue the same way and depend entirely on a proper cleaning routine to trust they're actually clean.
The trade-off is time, and a whirlpool trick that speeds it up
An immersion chiller typically cools a batch more slowly than a counterflow or plate design, since it's exchanging heat with a large, mostly-static volume of wort rather than a continuous flow — many homebrewers speed this up by gently stirring or whirlpooling the wort around the submerged coil as it chills, which keeps fresh hot wort moving past the coil's surface rather than relying on passive convection alone. A pre-chiller (a second coil sitting in an ice bath ahead of the main chiller) is a common further upgrade some brewers add.