Imagine that you have some toys, and you get to play with them whenever you like. But when you’re done with them, you give them to your friends to play with too, and they get to keep them forever.
That’s what public domain software is like! It’s like a computer program that someone made, and then they decided to share it with everyone else for free.
Usually, when someone makes a computer program, they can decide who is allowed to use it, and they can even charge money if they want to. But if they say the program is “public domain,” that means anyone can use it, and no one has to pay any money.
It’s like a giant toy box that everyone gets to play with, and no one has to fight over who gets to use what toys.