When you have a toy and you promise to let your friend borrow it, if you forget or decide not to give it to them, that’s like a default. It means you didn’t do what you said you would.
Imagine playing a game where you have to press a button to keep a balloon from flying away. If you don’t press the button, the balloon floats away. Not pressing the button is the default action because it’s what happens if you do nothing.
Think of a default setting on a computer or tablet. It’s like when your favorite game automatically starts with the same character every time unless you choose a different one. The character it starts with is the default choice.
In stories, a character might have a favorite ice cream flavor, like chocolate. If they always get chocolate unless they say otherwise, then chocolate is their default ice cream.
Sometimes, people have a default bedtime, like 8 PM. It means they go to bed at 8 unless they have permission to stay up later. So, a default is like the usual thing that happens unless you change it.