Green Squiggly Lines

I think computers and word processors (do they really process words, anyway?) have definitely changed the ways that people compose their writing. I think that just as there are ways to speak English (street slang, standard English, dialect) there are ways to write too, and I can't tell if the movers and shakers of the English community (ie professors, authors) have successfully reconciled this, nevermind grappling with the changing technology. What are green squiggly lines? The essay gets the point across without a lot of flashiness.