"Extends existing class on fly." What this means?!

Question

given in there.

Answer 1

"on the fly" means "while doing something else"

A person might call their spouse while flying home from a business trip. The phone call does not interrupt the trip, it's done "on the fly".

In the context of a computer program, the changes are made without interrupting / stopping the already-running program.

Answer 2

thanks :)