Its a relic from old days. Printers could only print 80 characters on a single line so text editors would draw a vertical line down the right side of a text area after where the 80th character would be.
Today however this is still used as a "general guideline" for writing code to prevent lines from getting too long. Also many UNIX terminals typically only show 80 columns so this guideline helps prevent ugly word wrapping.
As an example Eclipse has this feature, you can enable it by selecting General > Editors > Text Editors from the preferences window and checking "Show print margin". I've attached a photo of what it looks like.