Special effects are like that magic trick that a magician performs to amaze you. They are tricks used in movies and TV shows that make things look real, but they are not.
Imagine you are watching a movie about space, and you see a spaceship flying. The spaceship is not real, and it is not flying in the sky. It is made using computer graphics. A group of special effects artists uses their computers to create the spaceship and make it look like it is flying in space. They use complex software that can make anything possible.
Special effects can also be physical, not just made on a computer. For example, in a movie where someone is getting punched, they are not really getting punched. The actor is just pretending, and the punch sound is added later in post-production.
Another example could be the use of makeup to make actors look like monsters, aliens or anything out of this world.
In summary, special effects are the tricks that make movies and TV shows look cool, scary or exciting. They are the invisible magic that makes us believe that things are real when they are not.