Imagine there is a treasure chest filled with toys and candy called "The Pirate Bay." Lots of people want to play with these toys and eat the candy, but some grown-ups don't want them to because they think it's wrong to take things without paying for them.
These grown-ups are in charge of some countries, and they have decided to tell the computer machines in their countries to not let anyone go to "The Pirate Bay" and open the treasure chest.
So, if someone tries to go to "The Pirate Bay" website from one of these countries, their computer might say "Sorry, you can't come in here. This website is blocked."
The grown-ups hope that by doing this, people will stop trying to get the toys and candy from "The Pirate Bay" and start paying for them like they're supposed to.