Okay kiddo, imagine you have a bunch of toys and you want to give them to your friends, but you can only give them one toy at a time. That's kind of like how a serial digital interface works.
In your toys example, you give one toy to your friend, wait for them to take it, and then give them the next toy. That's how serial digital interface works too. It sends information one little piece, or bit, at a time.
But instead of toys, we're talking about things like pictures or sound from a computer or TV. The bits get sent through a wire or cable, just like how a toy can be handed over to your friend.
Now, imagine you have a picture with lots of colors, and you want to send it to your friend with a serial digital interface. Instead of sending all the colors at once (which would be too much information for the interface to handle), you send one color at a time until the whole picture is complete.
The good thing about serial digital interface is that even though it sends information slowly, it's very reliable and accurate. Just like how you can trust your best friend to take each toy carefully and not drop any, serial digital interface makes sure all the information gets sent in the right order and without any mistakes.