As you write a summary of your plot, you might notice that some of the words and names you've used require explanations. While Narnia, Hogwarts, and Gandalf may be household names now, before publication these names were complete gibberish. Now these words are part of beloved fictional worlds.
The more worldbuilding you've done, the more likely it is that you'll have had to make up words for places and things that don't exist in the real world. This technique is great inside your book, but it can really complicate a plot summary. Keeping your plot summary easy to understand is really important in a world where agents read fifty queries a day and readers are sorting through thousands of books online. Anything that confuses them or slows them down may make them lose interest in your book entirely. So how can you keep your plot summaries short and sweet when your story is full of made-up words?
Plots often revolve around conflict between named organizations or countries. As you explain the protagonist's goals, stakes, and conflict, these named groups may come up. However, it is generally best to replace these names with placeholders to keep your summary clear and short. Without the larger context of your book, readers of your summary will better understand terms such as "the rebels," "her hometown," "the villain's castle," etc. than the proper names of these people and places.
Every once and a while I've seen a story where a named organization or place plays a key role in the plot. If you find yourself referring to a named place or group more than three times in your summary, you may consider introducing the proper name. Concision is usually the best guide in these cases, so I recommend writing two versions of your summary and comparing them. In one, use the proper name, being sure to explain what it means the first tike it appears. In the other, use the placeholder all the way through. When you compare them, I recommend using whichever ends up shorter. Most of the time using a placeholder takes less words than both using and properly explaining a name, but repetition of a long placeholder may shift that balance.
In addition to naming places and groups of people, you may have had to create other kinds of words for your book, whether this is because the word refers to something that doesn't exist in our world (common in science fiction and fantasy stories) or because the real-world word didn't meet your story's needs. Whatever the reason, these made-up words can be great for worldbuilding, but can make a plot summary impossible to understand.
When talking about elements of your unique world, consider whether a real-world word conveys the meaning closely enough for the sake of the summary. If your world has a magical winged creature that is part human and part bird, you might have a cool name for them, but a reader can get the gist from "a winged person".
Remember that in summaries, readers only need to get the general idea of your story, not the detailed specifics. The real-world word may not work in the larger context of the story, but it can still be useful in the summary.
Sometimes these real-world replacements end up longer than the made-up word would be. Generally the replacements are still preferable because the made-up words carry no meaning for the readers. But if you find one made-up word coming up frequently in your plot summary, you may find it easier to introduce your word and then use that word for the rest of the summary. I recommend using no more than one invented word in a summary. Using more than one increases the odds that readers will get confused about the meaning of the word when it appears later in the summary.
Overall, that should leave you with maybe one named place or organization, and/or one invented noun. These words will also have been clearly explained the first time they appear in your summary. Following these guidelines should give you a manageable amount of new words for an agent, editor, or reader to absorb and still follow the plot points of your summary.
To help you find the right balance for your book, friends who aren't familiar with your book can be a huge help. These fresh eyes can easily identify any names and terms that will be confusing to an agent, editor, or reader. The more you can replace the confusing terms with common words, the better you will be able to keep the word count down and keep these plot summaries easy-to-read.
I have a separate post about handling character names in plot summaries that you may also find helpful.