Let's say you are reading a book about people with magical abilities. As a young child, their abilities manifest spontaneously (accidentally breaking something with their mind, flying instead of falling, etc). Then, they are taken away off somewhere to learn to hone their skills. There may be rules to what magic is and is not possible, how magic is done, and what different students can learn. In this book, the main character has the ability to manipulate electricity. They can use this to control anything that operates with electricity remotely, like by turning lights on and off, or they can just shock people. If it is established in the book that no person has more than one ability, you may be surprised if, later on, the main character starts to manipulate water or read minds. This changes the rules of the story.
In the above paragraph, I painted a picture of a universe, giving you rules about how it worked. If I asked you to write a story within the universe, many of you might take care to make sure the story abides by the rules I have provided. This ability to create a universe in your mind is a skill not everyone has, but it is useful beyond just reading stories and writing fanfiction. This same skill can be used to understand people, and even ourselves, better.
To tie this to another example, let's say you are speaking with someone who you just met. This person is an elementary school teacher. Right away, you know quite a few things about this person (they work with kids, get summers off, probably teach during the day and maybe grade papers and plan lessons after school). In the same way that you did with the book, you can use this information to give more detail to this person's universe. Now maybe the person tells you that they also run a summer camp. If you previously assumed that being a teacher meant they had summers off, now you can update the information you have on this person to include that they have a second job they work over the summer. You now have a slightly more detailed picture of what their universe is like.
Though the example above is a relatively simple one, there are lots of ways to gain information that you can use to add detail to someone's universe. Often times we don't even realize we are doing it; assumptions made based on a person's appearance or social media profile aren't always conscious. There are even small things people say and do that we miss, but could otherwise be useful information in understanding the person better. If someone tells you a movie they saw was "too scary", it may not be a fact about the movie, but about their dislike for horror movies, or fears surrounding the topic of the movie.
The perspective we use to analyze other universes is also important, because it determines what we notice and what we don't. When reading books, we may not think much of things that are normal for us, like a 6-year-old going to school for the first time, but notice things that are not true in our universe, like super powers. The more ways a fictional universe is different from our own, the harder it is to keep track of. Imagine how much easier it might be to follow a book about super heroes, rather than a book about aliens and monsters with magic and futuristic weapons! This is part of why we enjoy spending time with and talking with people who think similarly to us. If a friend's universe is similar to yours, it takes less effort to understand who they are and why they do what they do.
If you meet someone whose universe is too different from yours, you may find them hard to relate to. Part of this is because, by default, we use our own universe to look at other universes. A young child who doesn't know the meaning of divorce might have a hard time understanding the experiences of their friend, who spends half their time with Dad and half their time with Mom, but never together. The child may get frustrated if that friend leaves a book they borrowed at Dad's house, then didn't have it when they spent time together after school at Mom's. It can be so easy to judge other people without thinking about whether what happened makes sense in their universe if it doesn't make sense in yours. The child may assume that the friend didn't want to give the book back or was trying to be mean, rather than realize how easy it is to forget something at one house when you have two. Even adults do this all the time, assuming malice or stupidity when we can't understand another's actions. Taking care to understand another person's universe can help prevent this from happening, sometimes drawing attention to parts of your own universe that you take for granted in the process.