Jacketed vessel

Okay kiddo, imagine you have two cups, one that fits perfectly inside the other. The outer cup is like a special jacket with space between it and the inner cup. Now, let's say you want to keep your hot chocolate warm for a long time without burning your hands. What do you do? You pour your hot chocolate in the inner cup, and then fill the space between the two cups with hot water, just like a warm blanket. This way, the hot chocolate stays hot for longer, and since the outer cup is cooler than the inner cup, you can hold it without burning yourself.

That's what a jacketed vessel is like, with two layers of metal that have space between them that can be filled with hot or cold fluid, depending on what you're trying to do. Like heating up a liquid or keeping it cool, without burning or freezing what's inside the vessel. Pretty neat, huh?