Start talking to an avid horseback rider, and you'll find yourself surrounded with words that may seem unfamiliar. She'll talk about tacking up, hand and leg aids, the four gaits, bits, halters, horses who are head shy, making flying lead changes, and many other highly specific terms. Talk to a plumber or an electrician, or a mathematician or a field hockey player, and these folks will have the same degree of specialized words and phrases to describe the world of their profession. We call this type of specialized language "shop talk". Another term writers throw around to describe this is a lexicon. It's almost as though all these specialized worlds around us have their own secret codes, their own cryptologies, and we love to see it appear in fiction.
The specific is universal
The funny thing about really gripping writing, the type that everyone feels engaged by, is that it is usually highly specific. It looks at one small corner of the world, letting the reader see the ins and outs of that place, the language, the slang, the shop talk used. Somehow that specific look at one part of the world allows us to learn more about the world in general. Try to do the opposite, though, by being as broad, general, and vague as possible, and you'll appeal to no one. Your story will end up seeming false and not universal at all. What makes something seem realistic, therefore, is its specificity.
How to use a lexicon
To use a lexicon yourself in a story, you can first think about if you know any lexicons already. Pretty much everyone does; it might be the abbreviated, hyper-slangy lexicon of teenagers and textspeak, or the bland corporate neutrality of language spoken in the workplace. It might be the tone and word-rhythm of another language or culture, or the way a previous generation speaks. Or it might be the shop talk of your job. Whatever it is, it's a start, your in into another world. Think hard about what words and phrases characterize that lexicon and use them freely in your writing. The more you do, the more specific you'll be making your story.
It's always possible to learn a new lexicon in life, too. Try researching the world you want to write about -- but don't just research the official descriptions of that world. Instead, find someone who actually speaks the lexicon of that world and learn the popular slang. If you want to write about the world of a certain religion, for example, don't just write the way the priests talk; write the way the followers actually talk. Lexicons bring us news from another world, and that is precisely what is delightful about fiction.