Assume Nothing
While writing, never assume that every reader knows exactly what you know. You may be an expert in your subject, but your readers almost certainly won't all share the same knowledge.
Laymans Terms
Take science fiction as an example. Mentioning a flux capacitor may seem perfectly natural to you, but not everyone immediately knows what that is. Describe it instead as an engine that allows someone to travel through time, and almost every reader instantly understands the idea.
Once they understand the concept, introducing the technical term becomes much easier. The same principle applies whatever you write about.
Every Genre Has It's Own Language
Many readers enjoy a wide variety of books. I read everything from Iain M. Banks' science fiction to Charles Dickens, and most things in between. Other readers, however, remain loyal to a favourite genre and, over time, become familiar with its language, terminology and conventions.
A crime fiction reader may immediately understand DNA profiling, luminol or chain of custody. A legal thriller reader might recognise terms such as voir dire or amicus brief. Science fiction readers often come across ideas like ansatz, the Kardashev Scale, wormholes and singularities without giving them a second thought.
That doesn't mean every reader understands them.
Write For the New Reader
If you are writing for a science fiction audience, it is tempting to assume your readers already know what an ansatz is or where the Kardashev Scale fits into your story. Many will, some wont.
Writers are often advised not to explain everything to the reader, and that is usually sound advice. In this case, however, a brief explanation can make the difference between inviting someone into your story and leaving them behind.
It is a difficult balance to achieve. Read your work again, but this time imagine you are someone picking up their very first science fiction novel. Would you understand enough to enjoy the story? If not, your reader probably needs a little more guidance.
Final Thoughts
Never assume your readers know what you know. Give them just enough information to keep turning the page, and they will happily follow you wherever your story leads.