Button copy is the words that you see on a button when you use apps or websites. It helps you know what the button will do when you click on it. For example, the button that says "submit" on a form will make sure that the information you entered gets sent to the right place.
The people who design websites and apps choose the words to put on the buttons very carefully, so that it's clear to you what will happen if you click on them. They also try to make the words on the buttons easy to read and understand.
Sometimes, button copy can be tricky and confusing. That's why it's important to take a moment to read and make sure you understand what the button is for before you click on it, so that you don't accidentally do something you didn't mean to do.