An official script is like a set of rules on how to write things correctly. Just like how adults might tell you how to properly use your pencil or crayon to write your letters, an official script tells people how to write important things, like official documents or important letters.
For example, imagine if we only ever wrote letters or important papers in our own handwriting like we do when we write notes to our friends. That would be very confusing for other people to read, and it would make it hard for people to understand what the paper is talking about.
So, an official script is like a special type of handwriting that is used to make sure everyone who can read can understand what is written. It makes sure that all the letters and numbers are written the same way so that everyone can read them. It is like a secret code that everyone knows so they can communicate easily with each other.