So imagine you're playing with blocks, and you have different sizes and shapes of blocks. The Hille-Yosida Theorem is like a rule or formula that helps you sort those blocks out into different groups based on how they behave.
In math, we have different types of functions, kind of like different types of blocks. Sometimes, we want to sort those functions into groups based on how they behave when we do certain things to them. The Hille-Yosida Theorem helps us do that.
Basically, the theorem says that if we have a certain type of function, called a "bounded linear operator," and we want to know if it behaves nicely under certain conditions, like if we multiply it by another function, then we can use the Hille-Yosida Theorem to say whether or not it will behave nicely.
Think of it like this: if you have a blue block and a red block, and you want to know if you can stack them together without them falling over, you might use a rule that says "blue blocks can only be stacked on red blocks." That's kind of like what the Hille-Yosida Theorem does for math functions. It gives us a rule for how certain types of functions can be combined and still behave nicely.