In the mbox format for files with stored emails, a line starting with the string 'From ' is used to delimit messages.

This of course leads to problems if a line of one message itself starts with a 'From'. As a workaround, many standard Unix MUAs escape this line by prepending it with a '>', such that e.g.

From this day on, I could not find him anymore,
though I looked everywhere.
>From this day on, I could not find him anymore.
though I looked everywhere.
That way, the extra 'From's don't get in the way. The annoying consequence of course are the mangled lines in some mails. Some MUA try to reverse this escaping by removing the '>'s , but that's still a hack because then you can't have lines starting with '>From ' in your mails.

This all goes to show that the Unix mbox format is just not such a good design. Another attempt to solve the 'From' problem is the use of a Content-length header, which is arguably even worse.