The Merriam-Webster's Collegiate Dictionary (11th Edition) lists the first definition for jargon as confused, unintelligible language.
While this definition holds true of a lot of first drafts (and quite a few of this blog's posts, admittedly), the definition I want to focus on is "the technical terminology or characteristic idiom of a special activity or group."
What more special group is there than your novel's cast of characters? How often to you find yourself writing dialogue (or exposition) that makes complete sense to you, that falls well inside the normal speech patterns for your characters, only to hear from your beta-readers that they have no idea what your characters are talking about?
Especially when you're writing fantasy, unique terms and phrases to describe objects or states of being are necessary! But the problem is how to introduce those terms, that jargon, without throwing your reader into a tailspin of confusion as they try to decipher exactly what your characters are trying to say.
Once again, I turn to the talented J.K. Rowling to illustrate what I believe to be a top-notch example of how to work jargon into accepted language for the reader.
"Where was I?" said Hagrid, but at that moment, Uncle Vernon, still ashen-faced but looking very angry, moved into the firelight.
"He's not going," he said.
"I'd like ter see a great Muggle like you stop him," he said.
"A what?" said Harry, interested.
"A Muggle," said Hagrid, "it's what we call nonmagic folk like them. An' it's your bad luck you grew up in a family o' the biggest Muggles I ever laid eyes on."
If you do this too often, unfortunately, you're going to overwhelm your reader and cause them to fall out of sync with the story. But! For the important terms, it's worth experimenting with ways to sneak the explanation in.
One thing to note about the definition of the word muggle here and all it's nuances the importance of characterization in helping to paint the picture of the term. Rowling has spent chapters by this point characterizing the Dursleys and their relationship with Harry so that when Hagrid labels them muggles, the implications of such a word reach far beyond nonmagic. So much so that when you hear the word muggle, do you not immediately thing of Dursley?
Take some time and create a list of the jargon you employ to build your world. Which meanings are obvious to your readers? Which are creating unnecessary confusion? How can you craft your narrative in such a way that the meanings of the words stretch beyond the literal definitions?