Chromium -- commonly referred to simply as chrome -- is often used to coat other metals to provide a shiny, protective or decorative finish. While many objects such as aluminium motorcycle parts look shiny, that doesn't necessarily mean that they're chrome. Many metals like steel and brass are coated in chrome, often for a variety of different reasons. This is especially true for pipes, particularly those made of brass.
Brass pipes, like any other sort of metal that's coated with chromium, go through an electroplating process. This involves dipping the pipes in a solution that contains chromium in microscopic form. When the solution is excited by electricity, the chromium sticks to the brass pipes -- usually to the outside, as steps may be taken to stop the chromium from getting inside the pipe. This process results in a fairly even coating of chromium across the pipe, which many not be the case for other coatings that only look like chrome.
Chrome-coated brass pipes are resistant to corrosion. Chromium isn't a particularly strong metal by itself, but when it's coated over another metal it provides the base metal with an extra layer of protection against corrosion and oxidation. This is more easily seen in steel, which tends to rust when it oxidises, but brass pipes can also weaken as they corrode. Because of this particular property, chrome-plated brass pipes may be more expensive at first, but in the long run they tend to last much longer than straight brass pipes.
The most notable property of a chrome-plated brass pipe is the high shine. Polished aluminium and stainless steel are sometimes mistaken for chrome because of the metal's extremely shiny surface. Polishing chrome-plated brass pipes brings out even more sheen than chromium normally possesses. This is a purely cosmetic property, and it's more often viewed as a bonus to the chrome's corrosion fighting power than as a desirable property in itself.