Do you know what a coloring book is? It's a book with pictures where you can color inside the lines.
A filled Julia set is like a coloring book. Imagine a blank sheet of paper. Now, imagine that someone drew a really complicated shape on that paper.
Now, imagine that you can color the inside of the shape, but you can't color outside the lines. That's kind of what a filled Julia set is.
A mathematician named Gaston Julia discovered a type of math equation that can create very complicated shapes. These shapes are called Julia sets.
A filled Julia set is the area inside the shape of a Julia set, colored in. Some mathematicians like to study these shapes because they can help us understand how complicated things like weather patterns or the stock market work.
So, think of Julia sets like coloring book pages, and a filled Julia set is like when you color inside the lines of the page.