So, sovereigntism is like when you ask your mom if you can be in charge of your own room. Puerto Rico is an island in the ocean and it's part of a bigger country called the United States. Some people in Puerto Rico want to be in charge of their own island, like you want to be in charge of your room.
Right now, the United States makes most of the important decisions for Puerto Rico, like how much money the island gets and what kind of rules they have to follow. But some people in Puerto Rico think that it would be better if they made those decisions themselves.
Imagine if your mom said that you had to wear your winter coat all year round, even when it's really hot outside. You might think that's not fair, because you know your own body and you know when you're too hot or too cold. In the same way, some people in Puerto Rico think that they know what's best for their island, because they live there and they understand the problems that people face.
So to sum it up, sovereigntism in Puerto Rico is when some people on the island want to be in charge of their own destiny and make important decisions for themselves, instead of having the United States do it for them.