Here's how you can simplify complex concepts for non-technical individuals in game design.

The first step is to truly understand what the complex concepts are and mean. If you don't fully understand them then you can't really explain it well to someone. Then you want to find similar real-world examples. For example, if you want to explain what the difference between UI and UX is. You can something like UI is the visual parts of what the user interacts with. Like a book cover or the buttons on a DVD Player, TV, or Phone. It is the parts that look visually nice and you interact with it. The UX is how easily it is to do what you want, for example, the button on a DVD Player that says open, will open the DVD slot, the play button will start the video, or the power button is where it is easy to find and turns off the device.

Learn what UML diagrams are. You don't need to stick to all the rules when preparing diagrams for people unfamiliar with this modeling type, but having a general understanding of what a good diagram should include will help you greatly. A well-designed diagram can convey more information than a thousand words in much less time (and nobody likes to read documentation anyway, so don't make them).

The suggestion above is useful when communicating with people who don't speak the same industry-specific language as you do. However, "the characters in the game that you don't control" is much longer than "NPC," and too many words can cause problems too. Use simple concepts and the fewest words possible. Introducing too many new concepts, especially ones easily found online, can be as confusing as using complex language.

It is important to not use abbreviations without explaining the abbreviation first. For example, there is nothing wrong with say NPC stands for Non-Player Character, which is the computer-controlled characters. Some of these characters the player can interact with and some they can't. What is important is not to use fewer words or abbreviations but instead to clearly explain the concepts like the person is a child. Pretend you are explaining the concepts to a child, that doesn't mean being insulting or dumbing it down, more get to the point and be clear. If you practice explaining to a child then you will learn how to explain it with simple terms and references that anyone will understand. When in doubt, refer to #1 or #2.

For UX what is important is to explain how UX is the experience the player has with the controls and interface. For example, a good UX means that when a player wants to save the game, it is in the least amount of steps to do and it is clear what they need to press to get there. If there is an options menu, is the save feature in there, where players will expect it and maybe they need 3 button presses at most to save. One to open settings or options, one to open the save menu, and one to save their game. Also with controls what is important is the player can figure it out based on other games in the same genre without needing a manual. If X is jumping in all the other games at the same time, you should have X jump not Y. Intuitive is the key

Know your audience. Some people are more technical than others. Some are familiar with common terms while others are not. If the person is a visual learner, give them visual examples. Find out what their hobbies are, building a house, playing video games, reading, listening to audiobooks, etc.. Then use that in your analogies. If you cater it to something they know, it will be a lot easier to explain. What is important is to map what the audience understands, with something they don't to make it easier for them to understand. Then have them repeat back to you what you told them. And ask if that makes sense and if they understand. If they do, great, if they don't try to explain it using another example.