What does "subject line" mean?

Native speaker’s answer
Rebecca
The email subject line is the title of the email, aka the second line you see (it's found under the sender name). You can think of it as similar to being a title of a book, but for emails. Ex: McDonalds (Sender) Get your free drink on us today! (Subject line) Ex: Sir Douglas March (sender) You have won £1,000,000 in the lottery! (Subject line)