✔ 最佳答案
You usually hear this line in some older english movies. Usually, there is a newspaper boy waving a copy of the newspaper, saying "Extra, extra, read all about it."
Extra here means "特刊", an extra section that the newspaper prints out because of a sudden and important event or development.
Basically, the newspaper boy is anouncing there is some major news breaking out. By saying it out loud, he wants the people on the street to come and buy more newspaper from him.
Hope it helps.